Remove unnecessary checking in emacs-module.c

Remove unnecessary checking in emacs-module.c

* src/emacs-module.c (module_copy_string_contents):
Remove checking, as string lengths are always nonnegative and less
than STRING_BYTES_BOUND, and this is checked elsewhere.
(module_make_string): Check length against STRING_BYTES_BOUND, a
tighter bound than MOST_POSITIVE_FIXNUM.  (funcall_module): Don't
assume that an out-of-range integer is nonnegative.