return early instead of crashing if fileData fails

BUG=skia:
R=krajcevski@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/362833004
diff --git a/tests/KtxTest.cpp b/tests/KtxTest.cpp
index 83a3546..bddd09a 100644
--- a/tests/KtxTest.cpp
+++ b/tests/KtxTest.cpp
@@ -147,6 +147,9 @@
     SkBitmap etcBitmap;
     SkAutoTUnref<SkData> fileData(SkData::NewFromFileName(pkmFilename.c_str()));
     REPORTER_ASSERT(reporter, NULL != fileData);
+    if (NULL == fileData) {
+        return;
+    }
 
     bool installDiscardablePixelRefSuccess =
         SkInstallDiscardablePixelRef(