Detected by the clang static analyzer.

Bug: angleproject:5008
Change-Id: I773c8219b0c7ff63a2a98edefc5bebab7a9aabd5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2416829Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>