Consistently use validate_subarray to verify substring.

Consistently use validate_subarray to verify substring.
* fns.c (validate_substring): Not static any more.  Adjust to
use ptrdiff_t, not EMACS_INT, becase string and vector limits
can't exceed ptrdiff_t even if EMACS_INT is wider.
* lisp.h (validate_subarray): Add prototype.
* coding.c (Fundecodable_char_position):
* composite.c (Fcomposition_get_gstring, Fcompose_string_internal):
Use validate_subarray.  Adjust comment to mention substring.