Improve buffer-overflow checking.

Improve buffer-overflow checking.

* fileio.c (Finsert_file_contents):
* insdel.c (insert_from_buffer_1, replace_range, replace_range_2):
Remove the old (too-loose) buffer overflow checks.
They weren't needed, since make_gap checks for buffer overflow.
* insdel.c (make_gap_larger): Catch buffer overflows that were missed.
The old code merely checked for Emacs fixnum overflow, and relied
on undefined (wraparound) behavior.  The new code avoids undefined
behavior, and also checks for ptrdiff_t and/or size_t overflow.