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

[Ami Fischman]

More poking:

(gdb) p it->glyph_row->glyphs[1][20].object

$44 = 101922033

(gdb) xtype

Lisp_String

(gdb) xstring it->glyph_row->glyphs[1][20].object

$45 = (struct Lisp_String *) 0x61334f0

  ""

(gdb) p ((struct Lisp_String *)0x61334f0)->data

$48 = (unsigned char *) 0x4fe9388 ""

(gdb) p ((struct Lisp_String *)0x61334f0)->data[0]

$49 = 238 '\356'

(gdb) p ((struct Lisp_String *)0x61334f0)->data[1]

$50 = 128 '\200'

(gdb) p ((struct Lisp_String *)0x61334f0)->data[2]

$51 = 128 '\200'

(gdb) p ((struct Lisp_String *)0x61334f0)->data[3]

$52 = 0 '\000'