Fix callable variance check (GH-17613)

Caused by GH-15492. While the parent might contain callable, it may also
contain other types. zend_is_class_subtype_of_type() may be checking a
member that is not callable itself. Fall back to the normal class
subtype check.

Discovered by a failing Laravel test in nightly.

Zend/zend_inheritance.c

@@ -499,8 +499,6 @@ static inheritance_status zend_is_class_subtype_of_type(
 		} else if (fe_ce == zend_ce_closure) {
 			track_class_dependency(fe_ce, fe_class_name);
 			return INHERITANCE_SUCCESS;
-		} else {
-			return INHERITANCE_ERROR;
 		}
 	}