Fix crash when ETC1 codec is not supported.

BUG=skia:4216
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1567893002

Review URL: https://codereview.chromium.org/1567893002
diff --git a/gm/etc1bitmap.cpp b/gm/etc1bitmap.cpp
index d9b08d1..e2f6c2d 100644
--- a/gm/etc1bitmap.cpp
+++ b/gm/etc1bitmap.cpp
@@ -105,6 +105,10 @@
         }
 
         SkAutoTUnref<SkImage> image(SkImage::NewFromEncoded(fileData));
+        if (nullptr == image) {
+            SkDebugf("Could not decode the ETC file. ETC may not be included in this platform.\n");
+            return;
+        }
         canvas->drawImage(image, 0, 0);
     }