This can be null and this will causes a crash in strstr(). Check for null
before dereferencing this field.

BUG=angleproject:2475
TEST=angle_end2end_tests on Fuchsia and angle_enable_vulkan_validation_layers=true

Change-Id: I76a2738365838cb2f9cb54af7637417735762b69
Reviewed-on: https://chromium-review.googlesource.com/c/1446506Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>