ext/gmp: Fix GMP accepting integer strings with NUL bytes

[LamentXU123]

<?phpvar_dump(gmp_strval(gmp_init("123\0abc")));

string(3) "123"

The logic of dealing with argument position is copied from

php-src/ext/gmp/gmp.c

Lines 667 to 675 in e3de8ed

	if (-1==gmp_ret) {
	if (arg_pos==0) {
	zend_value_error("Number is not an integer string");
	} else {
	zend_argument_value_error(arg_pos, "is not an integer string");
	}
	returnFAILURE;
	}