gallium/radeon: handle buffer alloc failures in r600_draw_rectangle

Cc: 11.0 <mesa-stable@lists.freedesktop.org>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/drivers/radeon/r600_pipe_common.c

@@ -78,6 +78,9 @@ void r600_draw_rectangle(struct blitter_context *blitter,
 	 * I guess the 4th one is derived from the first 3.
 	 * The vertex specification should match u_blitter's vertex element state. */
 	u_upload_alloc(rctx->uploader, 0, sizeof(float) * 24, &offset, &buf, (void**)&vb);
+	if (!buf)
+		return;
+
 	vb[0] = x1;
 	vb[1] = y1;
 	vb[2] = depth;