Only update the surface size if a new buffer is attached

This fixes a regression caused by either 918f2dd4 or da75ee1d.  In
particular, if a client called commit without attaching a buffer and if the
compositor had already released its reference to the buffer, then a size of
0x0 would be set on the surface.  In particular, this affects the wayland
backend because it frequently sends only a frame request in order to cause
a refresh.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>

src/compositor.c

@@ -2110,10 +2110,10 @@ weston_surface_commit(struct weston_surface *surface)
 	surface->buffer_viewport = surface->pending.buffer_viewport;
 
 	/* wl_surface.attach */
-	if (surface->pending.buffer || surface->pending.newly_attached)
+	if (surface->pending.buffer || surface->pending.newly_attached) {
 		weston_surface_attach(surface, surface->pending.buffer);
-
-	weston_surface_set_size_from_buffer(surface);
+		weston_surface_set_size_from_buffer(surface);
+	}
 
 	if (surface->configure && surface->pending.newly_attached)
 		surface->configure(surface,
@@ -2336,12 +2336,12 @@ weston_subsurface_commit_from_cache(struct weston_subsurface *sub)
 	surface->buffer_viewport = sub->cached.buffer_viewport;
 
 	/* wl_surface.attach */
-	if (sub->cached.buffer_ref.buffer || sub->cached.newly_attached)
+	if (sub->cached.buffer_ref.buffer || sub->cached.newly_attached) {
 		weston_surface_attach(surface, sub->cached.buffer_ref.buffer);
+		weston_surface_set_size_from_buffer(surface);
+	}
 	weston_buffer_reference(&sub->cached.buffer_ref, NULL);
 
-	weston_surface_set_size_from_buffer(surface);
-
 	if (surface->configure && sub->cached.newly_attached)
 		surface->configure(surface, sub->cached.sx, sub->cached.sy);
 	sub->cached.sx = 0;