In some specific cases after binding a zero size buffer we could end
up trying to use a buffer storage that was no longer valid. Fix this
by ensuring we don't flush dirty bits when we have an early exit due
to a zero size buffer.

Bug: chromium:1107433
Change-Id: I9db560e8dd3699abed2bb7fe6d91060148ba1817
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2343405Reviewed-by: Jamie Madill <jmadill@chromium.org>