Fix check in GrGLGpu for whether PBO 0 is bound

Found in OOP-R canvas in Chrome. Most likely async read followed by
sync read with an intervening resetContext().

Bug: chromium:1208212

Change-Id: Ibf85f070d0a4cd389f67e9b249655b0ef667c66e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435277
Commit-Queue: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed