Clear temp buffer in GrContextPriv::readSurfacePixels2

This fixes an issue on Chrome's MSAN bots where glReadPixels into the
temp buffer doesn't actually overwrite the buffer.

Change-Id: I9cbdd208d3bdd0d53e96bbef1996648a3a21f66f
Reviewed-on: https://skia-review.googlesource.com/128013
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed