gpu fuzzers: AddRef on null pointers

Translators may be null (e.g. for passthrough, or in case of failure), so only
AddRef them if they are valid.

Bug: 779954
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I8f0039eaf2bdaf76a58e74317af68c13d252bdd5
Reviewed-on: https://chromium-review.googlesource.com/746993Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512894}

gpu/command_buffer/tests/fuzzer_main.cc

@@ -351,8 +351,13 @@ class CommandBufferSetup {
     // Keep a reference to the translators, which keeps them in the cache even
     // after the decoder is reset. They are expensive to initialize, but they
     // don't keep state.
-    decoder_->GetTranslator(GL_VERTEX_SHADER)->AddRef();
-    decoder_->GetTranslator(GL_FRAGMENT_SHADER)->AddRef();
+    scoped_refptr<gles2::ShaderTranslatorInterface> translator =
+        decoder_->GetTranslator(GL_VERTEX_SHADER);
+    if (translator)
+      translator->AddRef();
+    translator = decoder_->GetTranslator(GL_FRAGMENT_SHADER);
+    if (translator)
+      translator->AddRef();
     return decoder_->MakeCurrent();
   }