Fix the row number mistakenly reported by pos_visible_p in rare cases.

Fix the row number mistakenly reported by pos_visible_p in rare cases.

 src/xdisp.c (pos_visible_p): Properly save and restore the iterator
 state around the call to line_bottom, since it can move the
 iterator to another screen line.  This fixes off-by-one errors in
 the reported row in some rare cases.