Bug 1061891 - Make it possible to check whether a nsMainThreadPtr<T> is null. r=bholley

xpcom/glue/nsProxyRelease.h

@@ -154,6 +154,10 @@ public:
   {
     return mRawPtr == aOther.mRawPtr;
   }
+  bool operator!() const
+  {
+    return !mRawPtr;
+  }
 
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)
 
@@ -225,7 +229,9 @@ public:
     }
     return *mPtr == *aOther.mPtr;
   }
-  bool operator!() { return !mPtr; }
+  bool operator!() const {
+    return !mPtr || !*mPtr;
+  }
 };
 
 #endif