Tested and fixed compute_display_string_pos for buffer iteration.

Tested and fixed compute_display_string_pos for buffer iteration.
Also fixed an unrelated bug caused by vertical-motion.

 src/xdisp.c (Fcurrent_bidi_paragraph_direction): Initialize
 itb.string.lstring.
 (compute_display_string_pos, compute_display_string_end):
 Fix calculation of the object to scan.  Fixes an error when using
 arrow keys.
 (next_element_from_buffer): Don't abort when IT_CHARPOS is before
 base_level_stop; instead, set base_level_stop to BEGV.  Fixes
 crashes in vertical-motion.