Clarify aborted updateTexImage use of fences

When updateTexImage acquires a buffer but then aborts (due to an error
or the buffer being rejected), it releases the newly-acquired buffer.
It was passing the buffer slot's fences to releaseBuffer, even though
they hadn't been created after the acquire yet. This wasn't a bug,
since the fences would be cleared just after the buffer slot was last
released, but explicitly passing null fences makes this clearer.

Change-Id: I087f2ec3fd02c40f57782c1fca24eb9567e2943d