Prefer signed when testing for signed overflow

Prefer signed when testing for signed overflow

* src/alloc.c (free_cons):
* src/casefiddle.c (do_casify_multibyte_string):
* src/editfns.c (styled_format):
* src/image.c (png_load_body):
Use signed arguments to INT_MULTIPLY_WRAPV etc.  This doesn’t fix
any bugs, but GCC emits better code when all args are signed.
Also, this removes the need for an if in free_cons (Bug#37006).