ensure that we call onUnlock only when we onLock succeeded

src/lazy/SkDiscardablePixelRef.cpp

Index: src/lazy/SkDiscardablePixelRef.cpp
diff --git a/src/lazy/SkDiscardablePixelRef.cpp b/src/lazy/SkDiscardablePixelRef.cpp
index 6a9507c8c76a713bba3c7a85388e3f8e4c78d702..2528a994d185bb3e319785db5cb180bcdc03abc1 100644
--- a/src/lazy/SkDiscardablePixelRef.cpp
+++ b/src/lazy/SkDiscardablePixelRef.cpp
@@ -58,9 +58,7 @@ void* SkDiscardablePixelRef::onLockPixels(SkColorTable**) {
     return pixels;
 }
 void SkDiscardablePixelRef::onUnlockPixels() {
-    if (fDiscardableMemory != NULL) {
-        fDiscardableMemory->unlock();
-    }
+    fDiscardableMemory->unlock();
 }
 
 bool SkInstallDiscardablePixelRef(SkImageGenerator* generator,