Fixes assertion failures when ShaderGL fails to compile the generated shaders.

BUG=angleproject:882
BUG=angleproject:1149

Change-Id: Ib7c23a54bdb81852864d792f29c89644084859a3
Reviewed-on: https://chromium-review.googlesource.com/297085Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>