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