bug#12745: crash in bidi_pop_it during (idle) redisplay

[Alp Aker]

>> Yes, and so is the STRETCH glyph.  Can you figure out what is the
>> string before that, viz.:
>> >  20  144: CHAR[ ] str=61334f1[0] blev=0,btyp=L w=6 a+d=10+2 MB
>> This is a display string or an overlay string, but where does it come
>> from, and which mode puts it there?
>
>
> I'm groping in the dark, but AFAICT it's a null str?
> (gdb) p/x (it->glyph_row->glyphs[1][20].object)
> $39 = 0x61334f1
> (gdb) p *(it->glyph_row->glyphs[1][20].object)
> $40 = 0

That string is likely also from fci-mode.  The overlay string that
mode puts at the end of each line has the structure <character> +
<stretch glyph> + <image>, where by default <character> is U+E000.  It
shouldn't be null.