KHR_debug: Fix off-by-one message length. The dEQP tests were timing out due to the "get message" call skipping messages. The get call had a bufSize that ANGLE was rejecting as too small because it didn't include the null terminator. The dEQP test would pass but would always take ten seconds. I noticed this when debugging the ANGLE test launcher and seeing batch timeouts. Bug: angleproject:5130 Change-Id: Ie9f949218590b98a11f6c10b1f86074e90dbca17 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2447039 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>