Only update the surface size if a new buffer is attached

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>