Continue debugging continuation lines.

Continue debugging continuation lines.

 .gdbinit (prowlims, pmtxrows): New commands.
 xdisp.c (display_line): Special-case the calculation of row->end
 in rows that end at ZV, to avoid abort in CHAR_TO_BYTE.  Handle
 empty lines correctly when calculating row's max and min pos.
 Display cursor at the first glyph row that has its ends_at_zv_p
 flag set.