ensure that we call onUnlock only when we onLock succeeded

src/lazy/SkDiscardablePixelRef.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDiscardablePixelRef.h"
 #include "SkDiscardableMemory.h"
 #include "SkImageGenerator.h"
@@ skipping 40 matching lines @@
     if (NULL == fDiscardableMemory) {
         return NULL;  // Memory allocation failed.
     }
     void* pixels = fDiscardableMemory->data();
     if (!fGenerator->getPixels(fInfo, pixels, fRowBytes)) {
         return NULL;  // TODO(halcanary) Find out correct thing to do.
     }
     return pixels;
 }
 void SkDiscardablePixelRef::onUnlockPixels() {
-    if (fDiscardableMemory != NULL) {
-        fDiscardableMemory->unlock();
-    }
+    fDiscardableMemory->unlock();
 }
 
 bool SkInstallDiscardablePixelRef(SkImageGenerator* generator,
                                   SkBitmap* dst,
                                   SkDiscardableMemory::Factory* factory) {
     SkImageInfo info;
     SkASSERT(generator != NULL);
     if ((NULL == generator)
         || (!generator->getInfo(&info))
         || (!dst->setConfig(info, 0))) {
         SkDELETE(generator);
         return false;
     }
     SkASSERT(dst->config() != SkBitmap::kNo_Config);
     if (dst->empty()) { // Use a normal pixelref.
         SkDELETE(generator);  // Do not need this anymore.
         return dst->allocPixels(NULL, NULL);
     }
     SkAutoTUnref<SkDiscardablePixelRef> ref(SkNEW_ARGS(SkDiscardablePixelRef,
                                                    (generator, info,
                                                     dst->getSize(),
                                                     dst->rowBytes(),
                                                     factory)));
     dst->setPixelRef(ref);
     return true;
 }