fix warning: make impossible condition explicit

Change-Id: Ib96b9f117e8a18d91b5e3470f82186dbe84bd320
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

src/shared/proparser/qmakeevaluator.cpp

@@ -635,6 +635,7 @@ QMakeEvaluator::VisitReturn QMakeEvaluator::visitProBlock(
                     continue;
             }
             Q_ASSERT_X(false, "visitProBlock", "unexpected item type");
+            continue;
         }
         if (ret != ReturnTrue && ret != ReturnFalse)
             return ret;