It was wrong to use vararg after vsnprintf() affected it.
Luckily, we don't need to call vsnprintf() on vararg,
since the previous call of vsnprintf() on varargCopy
already gives us the length that we need.

Bug: angleproject:5131
Change-Id: Ie9b62e92ef8ab7e06b51e034c99a5fde20c1ceaf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2453930Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>