added null ptr check (fix bug 16959)

src/mesa/main/context.c

@@ -711,7 +711,8 @@ delete_renderbuffer_cb(GLuint id, void *data, void *userData)
 {
    struct gl_renderbuffer *rb = (struct gl_renderbuffer *) data;
    rb->RefCount = 0;  /* see comment for FBOs above */
-   rb->Delete(rb);
+   if (rb->Delete)
+      rb->Delete(rb);
 }