Bug 658909 - Initialize the same fields in both XPCCallContext constructors. r=mrbkap

This is a huge footgun. XPCCallContext is hot, but it's not too hot to be safe
here. Garbage XPCWN and JSObject pointers are bad.

js/xpconnect/src/XPCCallContext.cpp

@@ -26,7 +26,10 @@ XPCCallContext::XPCCallContext(XPCContext::LangType callerLanguage,
         mJSContext(cx),
         mContextPopRequired(false),
         mDestroyJSContextInDestructor(false),
-        mCallerLanguage(callerLanguage)
+        mCallerLanguage(callerLanguage),
+        mFlattenedJSObject(nullptr),
+        mWrapper(nullptr),
+        mTearOff(nullptr)
 {
     Init(callerLanguage, callerLanguage == NATIVE_CALLER, obj, funobj,
          INIT_SHOULD_LOOKUP_WRAPPER, name, argc, argv, rval);