This fixes a very odd use case where an app would try to copy from an
uninitialized buffer. I didn't search the spec too closely, but it's
likely a valid operation that produces undefined buffer contents.
Previously to this change we would genearte an OOM error.

Also includes an unrelated fix to ensure the latest buffer storage is
never nullptr when we have any data.

Bug: angleproject:1155
Change-Id: I4292bd302cc2b84d125a7d3e8d28e4d2b0210e53
Reviewed-on: https://chromium-review.googlesource.com/774991Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>