diffviewer: stop reading past the end of a line, and free the word diff

[ilia-maslakov]

Problem

Two defects in the diff viewer, both reached by simply opening a diff.

dview_str_utf8_offset_to_pos() reads past the end of the line. It is asked
for the byte offset of a given column, and the column it is asked for is the
width of the pane. A line shorter than that is walked past the end:
g_utf8_offset_to_pointer() counts out the characters it is asked for without
looking for the end of the string.

Invalid read of size 1
   at 0x4CE8C18: g_utf8_offset_to_pointer
   by 0x1A919A: dview_str_utf8_offset_to_pos (ydiff.c:623)
   by 0x1AC4DA: dview_display_file (ydiff.c:2602)
   by 0x1ACC10: dview_update (ydiff.c:2799)

hdiff_multi() drops the array of common substrings. ret is freed only
where the function found something and every recursion below it succeeded. The
ordinary case — nothing in common, which is how the recursion ends — falls
through to the bottom of the function and loses it, as do the three
return FALSE paths.

Change

Past the end of the line the rest is padding, one byte to a column, which is the
number g_utf8_offset_to_pointer() was arriving at anyway by stepping over
whatever followed the string; it is now computed instead of walked to. The
rendering is unchanged — I compared the screen dumps of a diff before and after,
byte for byte.

hdiff_multi() keeps the result in a local and frees ret on the way out of
every path.

Verified

Opening a diff, scrolling it, switching the layout and quitting, under
valgrind --leak-check=full --errors-for-leak-kinds=definite:

	invalid reads	leaked blocks
before	8	9
after	0	0

Ubuntu 24.04, --enable-werror: no compiler warnings, make check 69/69.

[mc-worker]

dview_str_utf8_offset_to_pos is similar to str_utf8_offset_to_pos. We should str_utf8_offset_to_pos in diffviewer and fix it not dview_str_utf8_offset_to_pos.
I created 5153_diffview_stop_read_after_eol branch with such modifications.