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/+/2076137Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>