mp_obj_is_callable(): Only object types can be callable.

Fixes segfault on callable("string").

py/obj.c

@@ -66,7 +66,7 @@ void mp_obj_print_exception(mp_obj_t exc) {
 }
 
 bool mp_obj_is_callable(mp_obj_t o_in) {
-    if (MP_OBJ_IS_SMALL_INT(o_in)) {
+    if (!MP_OBJ_IS_OBJ(o_in)) {
         return false;
     } else {
         mp_obj_base_t *o = o_in;

tests/basics/builtin-callable.py

@@ -0,0 +1,5 @@
+import sys
+print(callable(1))
+print(callable("dfsd"))
+print(callable(callable))
+print(callable(sys))