ensure that we call onUnlock only when we onLock succeeded

src/lazy/SkCachingPixelRef.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 "SkCachingPixelRef.h"
 #include "SkScaledImageCache.h"
 
@@ skipping 72 matching lines @@
     // destroy bitmap.  The *only* guarantee that this pointer
     // remains valid is the guarantee made by
     // SkScaledImageCache that it will not destroy the *other*
     // bitmap (SkScaledImageCache::Rec.fBitmap) that holds a
     // reference to the concrete PixelRef while this record is
     // locked.
     return pixels;
 }
 
 void SkCachingPixelRef::onUnlockPixels() {
-    if (fScaledCacheId != NULL) {
-        SkScaledImageCache::Unlock(
-            static_cast<SkScaledImageCache::ID*>(fScaledCacheId));
-        fScaledCacheId = NULL;
-    }
+    SkASSERT(fScaledCacheId != NULL);
+    SkScaledImageCache::Unlock( static_cast<SkScaledImageCache::ID*>(fScaledCacheId));
+    fScaledCacheId = NULL;
 }