Fix for bug 171121. prevValue could be null therefore we need to use NS_IF_RELEASE instead of

NS_RELEASE. R=dougt, SR=sspitzer

xpcom/ds/nsProperties.cpp

@@ -116,7 +116,7 @@ nsProperties::Set(const char* prop, nsISupports* value)
     nsCStringKey key(prop);
 
     nsISupports* prevValue = (nsISupports*)Put(&key, value);
-    NS_RELEASE(prevValue);
+    NS_IF_RELEASE(prevValue);
     NS_IF_ADDREF(value);
     return NS_OK;
 }