Bug 1430550 - Don't keep mutex locked if SourceSurfaceSkia::Map fails. r=bas

This was leading to a mutex never being unlocked, eventually causing a
crash when it was destroyed.

MozReview-Commit-ID: JzEDWzKxZ4S

--HG--
extra : rebase_source : c86ad738addbec55f33165d300876f4c675cf5f4

gfx/2d/SourceSurfaceSkia.cpp

@@ -170,6 +170,9 @@ SourceSurfaceSkia::Map(MapType, MappedSurface *aMappedSurface)
   aMappedSurface->mData = GetData();
   aMappedSurface->mStride = Stride();
   mIsMapped = !!aMappedSurface->mData;
+  if (!mIsMapped) {
+    mChangeMutex.Unlock();
+  }
   return mIsMapped;
 }