util: object: Reset pointer when unrefing object in virObjectAutoUnref

The helper function is used by the VIR_AUTOUNREF macro. Prior art is to
clear the pointer even if the variable goes out of scope.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/util/virobject.c

@@ -372,6 +372,7 @@ virObjectAutoUnref(void *objptr)
 {
     virObjectPtr *obj = objptr;
     virObjectUnref(*obj);
+    *obj = NULL;
 }