bug#37385: 27.0.50; Crash on multibyte assertion violation

[Juri Linkov <juri@linkov.net>]

Testing the tabs branch helped to expose a bug in master:

In GNU Emacs 27.0.50 (build 6, x86_64-pc-linux-gnu)
 of 2019-09-11 built on localhost
Repository revision: 4d90fadf27ccbb98e0e174304cb4e3008bf364fc
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Linux Mint 19.1

Configured using:
 'configure --with-x-toolkit=no --enable-checking=yes,glyphs
 --enable-check-lisp-object-type 'CFLAGS=-O0 -g3''

These steps reproduce the crash in master:

0. emacs -Q
1. Eval: (define-key global-map [menu-bar test] '("Test ⮿" keymap))
2. Visit an image file, e.g. etc/images/attach.pbm

#0  0x00005555557a61b8 in terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:374
#1  0x00005555558c7555 in die (msg=0x555555ae73ee "SINGLE_BYTE_CHAR_P (c)", file=0x555555ae4790 "xdisp.c", line=7250) at alloc.c:7256
#2  0x00005555555e9e56 in get_next_display_element (it=0x7fffffff89b0) at xdisp.c:7250
#3  0x000055555562938c in display_string (string=0x0, lisp_string=XIL(0x5555567a0204), face_string=XIL(0), face_string_pos=0, start=0, it=0x7fffffff89b0, field_width=7, precision=0, max_x=674, multibyte=-1) at xdisp.c:25489
#4  0x00005555556239db in display_menu_bar (w=0x5555565cae40) at xdisp.c:23533
#5  0x000055555560d61e in redisplay_window (window=XIL(0x5555565cae45), just_this_one_p=false) at xdisp.c:17821
#6  0x0000555555603b79 in redisplay_window_0 (window=XIL(0x5555565cae45)) at xdisp.c:15116
#7  0x0000555555921b23 in internal_condition_case_1 (bfun=0x555555603b37 <redisplay_window_0>, arg=XIL(0x5555565cae45), handlers=XIL(0x7fffe995aad3), hfun=0x555555603aff <redisplay_window_error>) at eval.c:1379
#8  0x0000555555603ad1 in redisplay_windows (window=XIL(0x5555565cae45)) at xdisp.c:15096
#9  0x00005555556024c2 in redisplay_internal () at xdisp.c:14579
#10 0x00005555555ffea3 in redisplay () at xdisp.c:13806
#11 0x00005555557b7948 in read_char (commandflag=1, map=XIL(0x555556a2aba3), prev_event=XIL(0), used_mouse_menu=0x7fffffffdd25, end_time=0x0) at keyboard.c:2472
#12 0x00005555557c930f in read_key_sequence (keybuf=0x7fffffffdf10, prompt=XIL(0), dont_downcase_last=false, can_return_switch_frame=true, fix_current_buffer=true, prevent_redisplay=false) at keyboard.c:9125
#13 0x00005555557b3d8b in command_loop_1 () at keyboard.c:1345
#14 0x0000555555921a48 in internal_condition_case (bfun=0x5555557b390d <command_loop_1>, handlers=XIL(0x90), hfun=0x5555557b2ed7 <cmd_error>) at eval.c:1355
#15 0x00005555557b34f4 in command_loop_2 (ignore=XIL(0)) at keyboard.c:1091
#16 0x0000555555920ea2 in internal_catch (tag=XIL(0xcdb0), func=0x5555557b34c7 <command_loop_2>, arg=XIL(0)) at eval.c:1116
#17 0x00005555557b3492 in command_loop () at keyboard.c:1070
#18 0x00005555557b29be in recursive_edit_1 () at keyboard.c:714
#19 0x00005555557b2bb6 in Frecursive_edit () at keyboard.c:786
#20 0x00005555557a8acc in main (argc=3, argv=0x7fffffffe368) at emacs.c:2086

An assertion violation is in get_next_display_element:

	  if (! it->multibyte_p && ! ASCII_CHAR_P (c))
	    {
	      eassert (SINGLE_BYTE_CHAR_P (c));

The menu item uses a multibyte char:

  c = 11199 (#o25677, #x2bbf, ?⮿)

But init_iterator sets multibyte_p in the menu-bar window to the
value of enable-multibyte-characters in the current buffer where
enable-multibyte-characters is nil when an image file is visited in
image-mode:

  /* Are multibyte characters enabled in current_buffer?  */
  it->multibyte_p = !NILP (BVAR (current_buffer, enable_multibyte_characters));

I tried the following fix and it prevents the crash:

diff --git a/src/xdisp.c b/src/xdisp.c
index 94f969f37c..5730145268 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -2984,7 +2984,9 @@ init_iterator (struct it *it, struct window *w,
   it->dp = window_display_table (w);
 
   /* Are multibyte characters enabled in current_buffer?  */
-  it->multibyte_p = !NILP (BVAR (current_buffer, enable_multibyte_characters));
+  it->multibyte_p = WINDOW_MENU_BAR_P (w)
+    || WINDOW_TOOL_BAR_P (w)
+    || !NILP (BVAR (current_buffer, enable_multibyte_characters));
 
   /* Get the position at which the redisplay_end_trigger hook should
      be run, if it is to be run at all.  */
@@ -6864,7 +6866,9 @@ reseat_1 (struct it *it, struct text_pos pos, bool set_stop_p)
   it->method = GET_FROM_BUFFER;
   it->object = it->w->contents;
   it->area = TEXT_AREA;
-  it->multibyte_p = !NILP (BVAR (current_buffer, enable_multibyte_characters));
+  it->multibyte_p = WINDOW_MENU_BAR_P (it->w)
+    || WINDOW_TOOL_BAR_P (it->w)
+    || !NILP (BVAR (current_buffer, enable_multibyte_characters));
   it->sp = 0;
   it->string_from_display_prop_p = false;
   it->string_from_prefix_prop_p = false;