Fix CVE-2016-8674 (use-after-free in pdf_to_num()).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-8674
https://security-tracker.debian.org/tracker/CVE-2016-8674

Patch copied from upstream source repository:
http://git.ghostscript.com/?p=mupdf.git;h=1e03c06456d997435019fb3526fa2d4be7dbc6ec

diff --git a/include/mupdf/pdf/document.h b/include/mupdf/pdf/document.h
index aabf05f..0078c4a 100644
--- a/include/mupdf/pdf/document.h
+++ b/include/mupdf/pdf/document.h
@@ -269,6 +269,10 @@ struct pdf_document_s
 		fz_hash_table *images;
 		fz_hash_table *fonts;
 	} resources;
+
+	int orphans_max;
+	int orphans_count;
+	pdf_obj **orphans;
 };
 
 /*
diff --git a/include/mupdf/pdf/object.h b/include/mupdf/pdf/object.h
index 5bc3dca..bf57455 100644
--- a/include/mupdf/pdf/object.h
+++ b/include/mupdf/pdf/object.h
@@ -110,6 +110,7 @@ pdf_obj *pdf_dict_gets(fz_context *ctx, pdf_obj *dict, const char *key);
 pdf_obj *pdf_dict_getsa(fz_context *ctx, pdf_obj *dict, const char *key, const char *abbrev);
 void pdf_dict_put(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val);
 void pdf_dict_put_drop(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val);
+void pdf_dict_get_put_drop(fz_context *ctx, pdf_obj *dict, pdf_obj *key, pdf_obj *val, pdf_obj **old_val);
 void pdf_dict_puts(fz_context *ctx, pdf_obj *dict, const char *key, pdf_obj *val);
 void pdf_dict_puts_drop(fz_context *ctx, pdf_obj *dict, const char *key, pdf_obj *val);
 void pdf_dict_putp(fz_context *ctx, pdf_obj *dict, const char *path, pdf_obj *val);
diff --git a/source/pdf/pdf-object.c b/source/pdf/pdf-object.c
index b4e33f3..1c19ba4 100644
--- a/source/pdf/pdf-object.c
+++ b/source/pdf/pdf-object.c
@@ -1265,11 +1265,14 @@ pdf_dict_geta(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *abbrev)
 	return pdf_dict_get(ctx, obj, abbrev);
 }
 
-void
-pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
+static void
+pdf_dict_get_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val, pdf_obj **old_val)
 {
 	int i;
 
+	if (old_val)
+		*old_val = NULL;
+
 	RESOLVE(obj);
 	if (!OBJ_IS_DICT(obj))
 		fz_throw(ctx, FZ_ERROR_GENERIC, "not a dict (%s)", pdf_objkindstr(obj));
@@ -1295,7 +1298,10 @@ pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
 		{
 			pdf_obj *d = DICT(obj)->items[i].v;
 			DICT(obj)->items[i].v = pdf_keep_obj(ctx, val);
-			pdf_drop_obj(ctx, d);
+			if (old_val)
+				*old_val = d;
+			else
+				pdf_drop_obj(ctx, d);
 		}
 	}
 	else
@@ -1316,10 +1322,27 @@ pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
 }
 
 void
+pdf_dict_put(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
+{
+	pdf_dict_get_put(ctx, obj, key, val, NULL);
+}
+
+void
 pdf_dict_put_drop(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val)
 {
 	fz_try(ctx)
-		pdf_dict_put(ctx, obj, key, val);
+		pdf_dict_get_put(ctx, obj, key, val, NULL);
+	fz_always(ctx)
+		pdf_drop_obj(ctx, val);
+	fz_catch(ctx)
+		fz_rethrow(ctx);
+}
+
+void
+pdf_dict_get_put_drop(fz_context *ctx, pdf_obj *obj, pdf_obj *key, pdf_obj *val, pdf_obj **old_val)
+{
+	fz_try(ctx)
+		pdf_dict_get_put(ctx, obj, key, val, old_val);
 	fz_always(ctx)
 		pdf_drop_obj(ctx, val);
 	fz_catch(ctx)
diff --git a/source/pdf/pdf-repair.c b/source/pdf/pdf-repair.c
index 690bf15..167f609 100644
--- a/source/pdf/pdf-repair.c
+++ b/source/pdf/pdf-repair.c
@@ -260,6 +260,27 @@ pdf_repair_obj_stm(fz_context *ctx, pdf_document *doc, int stm_num)
 	}
 }
 
+static void
+orphan_object(fz_context *ctx, pdf_document *doc, pdf_obj *obj)
+{
+	if (doc->orphans_count == doc->orphans_max)
+	{
+		int new_max = (doc->orphans_max ? doc->orphans_max*2 : 32);
+
+		fz_try(ctx)
+		{
+			doc->orphans = fz_resize_array(ctx, doc->orphans, new_max, sizeof(*doc->orphans));
+			doc->orphans_max = new_max;
+		}
+		fz_catch(ctx)
+		{
+			pdf_drop_obj(ctx, obj);
+			fz_rethrow(ctx);
+		}
+	}
+	doc->orphans[doc->orphans_count++] = obj;
+}
+
 void
 pdf_repair_xref(fz_context *ctx, pdf_document *doc)
 {
@@ -528,12 +549,13 @@ pdf_repair_xref(fz_context *ctx, pdf_document *doc)
 			/* correct stream length for unencrypted documents */
 			if (!encrypt && list[i].stm_len >= 0)
 			{
+				pdf_obj *old_obj = NULL;
 				dict = pdf_load_object(ctx, doc, list[i].num);
 
 				length = pdf_new_int(ctx, doc, list[i].stm_len);
-				pdf_dict_put(ctx, dict, PDF_NAME_Length, length);
-				pdf_drop_obj(ctx, length);
-
+				pdf_dict_get_put_drop(ctx, dict, PDF_NAME_Length, length, &old_obj);
+				if (old_obj)
+					orphan_object(ctx, doc, old_obj);
 				pdf_drop_obj(ctx, dict);
 			}
 		}
diff --git a/source/pdf/pdf-xref.c b/source/pdf/pdf-xref.c
index 7d21775..0cf20d4 100644
--- a/source/pdf/pdf-xref.c
+++ b/source/pdf/pdf-xref.c
@@ -1620,6 +1620,12 @@ pdf_drop_document_imp(fz_context *ctx, pdf_document *doc)
 
 		pdf_drop_resource_tables(ctx, doc);
 
+		for (i = 0; i < doc->orphans_count; i++)
+		{
+			pdf_drop_obj(ctx, doc->orphans[i]);
+		}
+		fz_free(ctx, doc->orphans);
+
 		fz_free(ctx, doc);
 	}
 	fz_always(ctx)
-- 
2.9.1