Fail gracefully on PBO pack buffer failure.

In certain error conditions we could call glReadPixels with both
a pack buffer bound and using a client pointer argument, which
the GL would interpret as a large buffer offset.

BUG=340694

Review URL: https://codereview.chromium.org/153953004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254332 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254332