We can't trust the reply from Xcb if an error has been flagged.
This was causing a crash in SwANGLE.

Bug: chromium:1080984
Change-Id: Icaf58197869b563e4193efcf4e70646d519210cf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2209255Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>