AURORA: Create copies in ASBuffer::actionGreater()

Popping the stack potentially invalidates the references to the top,
so we need to copy the variable before popping.

This fixes Coverity Scan issue #1403729.

src/aurora/actionscript/asbuffer.cpp

@@ -529,9 +529,9 @@ void ASBuffer::actionEnumerate2() {
 }
 
 void ASBuffer::actionGreater() {
-	const Variable &a = _stack.top();
+	const Variable a = _stack.top();
 	_stack.pop();
-	const Variable &b = _stack.top();
+	const Variable b = _stack.top();
 	_stack.pop();
 
 	_stack.push(a < b);