Assertion failure in set_iterator_to_next

[Stefan Monnier <monnier@iro.umontreal.ca>]

While playing with my nhexl-mode.el I bumped into some assertion failure
in the redisplay code.  More specifically the assertion

	  xassert (IT_BYTEPOS (*it) == CHAR_TO_BYTE (IT_CHARPOS (*it)));

at line 6183.
I think the problem is that the non-bidi code above that line uses
it->len assuming it holds the length of the char at point (well, at
IT_CHARPOS(it)), whereas IIUC it holds the length of "the" glyph.
In my case it->c holds sometimes the "lf symbol" char (u240a) which comes
from a display property and it->len is then 3, which is obviously not
the length of the char at IT_CHARPOS since the buffer I was looking at
only happens to contain ASCII chars.

I use the quick patch below, which seems to fix the problem, but I don't
know enough of the redisplay to know whether that's the right fix, or
whether the right fix is to make sure it->len holds some other value.


        Stefan


--- src/xdisp.c 2010-04-06 16:00:42 +0000
+++ src/xdisp.c 2010-04-10 01:35:11 +0000
@@ -6151,7 +6151,8 @@
 
          if (!it->bidi_p)
            {
-             IT_BYTEPOS (*it) += it->len;
+             IT_BYTEPOS (*it)
+               += BYTES_BY_CHAR_HEAD (*BUF_BYTE_ADDRESS (XBUFFER (it->object), IT_BYTEPOS (*it)));
              IT_CHARPOS (*it) += 1;
            }
          else