Fix error in osmcallback hook

The callback hook should use the old pointer instead of the new.
This causes a segfault when ptr is NULL.

.unreleased/PR_6106

@@ -0,0 +1 @@
+Fixes: #6106 incorrect pointer usage in OSM hook callback

src/osm_callbacks.c

@@ -61,7 +61,7 @@ ts_get_osm_hypertable_drop_hook()
 	{
 		OsmCallbacks *ptr_old = ts_get_osm_callbacks_old();
 		if (ptr_old)
-			return ptr->hypertable_drop_hook;
+			return ptr_old->hypertable_drop_hook;
 	}
 	return NULL;
 }