Misc: Don't run etc1 bench with invalid image and fix VS2015 build warning

Review URL: https://codereview.chromium.org/1409993003
diff --git a/bench/ETCBitmapBench.cpp b/bench/ETCBitmapBench.cpp
index d4068ca..fe6fe06 100644
--- a/bench/ETCBitmapBench.cpp
+++ b/bench/ETCBitmapBench.cpp
@@ -123,7 +123,7 @@
         : fDecompress(decompress), fBackend(backend) { }
 
     bool isSuitableFor(Backend backend) override {
-        return backend == this->fBackend;
+        return SkToBool(fImage) && backend == this->fBackend;
     }
 
 protected: