Fix rendering of composed caharacters on the mode line.  (Bug#19435)

 src/xdisp.c (set_iterator_to_next) <GET_FROM_STRING>: Limit search in
 composition_compute_stop_pos to the number of characters in the
 string.
 <GET_FROM_BUFFER, GET_FROM_STRING>: Simplify code.
 src/composite.c (composition_compute_stop_pos): If no composition
 was found in a string before ENDPOS, and ENDPOS is the string end,
 no need to back up to a safe point.
 src/dispextern.h (struct it) <end_charpos>: Improve commentary.