Use valSeen since it results in unused warning otherwise

Change-Id: Iccfa4b895d9e50227efa1747ea20ce07ce70327e
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>

src/testlib/qbenchmarkvalgrind.cpp

@@ -122,7 +122,8 @@ qint64 QBenchmarkValgrindUtils::extractResult(const QString &fileName)
             break;
         }
     }
-    Q_ASSERT(valSeen);
+    if (!valSeen)
+        qFatal("Failed to extract result");
     return val;
 }