Fix MultipleDrawBuffers sample. Was attempting to read invalid shader paths. Noticed when working on trace/replay. Bug: angleproject:4433 Change-Id: If43c08891b40f210112db8ffb98b8c6f53528496 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2076137 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>