bug 910376 - don't risk leaving mSharedBlobData uninitialized. r=karlt

gfx/thebes/gfxFont.h

@@ -604,12 +604,20 @@ private:
         typedef KeyClass::KeyTypePointer KeyTypePointer;
 
         FontTableHashEntry(KeyTypePointer aTag)
-            : KeyClass(aTag), mBlob() { }
+            : KeyClass(aTag)
+            , mSharedBlobData(nullptr)
+            , mBlob(nullptr)
+        { }
 
+        // NOTE: This assumes the new entry belongs to the same hashtable as
+        // the old, because the mHashtable pointer in mSharedBlobData (if
+        // present) will not be updated.
         FontTableHashEntry(FontTableHashEntry&& toMove)
             : KeyClass(mozilla::Move(toMove))
+            , mSharedBlobData(mozilla::Move(toMove.mSharedBlobData))
             , mBlob(mozilla::Move(toMove.mBlob))
         {
+            toMove.mSharedBlobData = nullptr;
             toMove.mBlob = nullptr;
         }