Fix gcguard_delete for rb_objspace_call_finalizer

rb_objspace_call_finalizer calls dfree functions in the order in which
the objects are lined up on the heap.  So the gcguard object may be
freed before the objects guarded in it.  This change avoids the
TypeError occurred on TypedData_Get_Struct in such the case.

Author: mrkn

ext/pycall/gc.c

@@ -57,13 +57,16 @@ gcguard_aset(VALUE gcguard, PyObject *pyptr, VALUE rbobj)
 static void
 gcguard_delete(VALUE gcguard, PyObject *pyptr)
 {
-  struct gcguard *gg;
-  st_data_t key, val;
+  if (rb_typeddata_is_kind_of(gcguard, &gcguard_data_type)) {
+    /* This check is necessary to avoid error on the process finalization phase */
+    struct gcguard *gg;
+    st_data_t key, val;
 
-  TypedData_Get_Struct(gcguard, struct gcguard, &gcguard_data_type, gg);
+    TypedData_Get_Struct(gcguard, struct gcguard, &gcguard_data_type, gg);
 
-  key = (st_data_t)pyptr;
-  st_delete(gg->guarded_objects, &key, &val);
+    key = (st_data_t)pyptr;
+    st_delete(gg->guarded_objects, &key, &val);
+  }
 }
 
 static ID id_gcguard_table;