CPlusPlus: Fix handling incomplete macro invocations

It looks very much as though the original author simply forgot to add
that return statement.

Fixes: QTCREATORBUG-23881
Change-Id: Ie93d2451bf1b491d01137285f983d657133c81c3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/cplusplus/pp-engine.cpp

@@ -1557,7 +1557,8 @@ bool Preprocessor::collectActualArguments(PPToken *tk, QVector<QVector<PPToken>
     }
 
     if (!tk->is(T_RPAREN)) {
-        //###TODO: else error message
+        return false;
+        //###TODO: error message
     }
     return true;
 }