gpu: Early out earlier for bad target in RasterDecoder

GetLevelSize calls GLTargetToFaceIndex which asserts if the target is bogus, so move
the early out before this call.

Bug: 868242
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I24a6262e2f7cf3ebf70c814bfca818365ec6c242
Reviewed-on: https://chromium-review.googlesource.com/1158801
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579909}
1 file changed