Add missing assertion to QAbstractTestLogger.

If passed an empty string, QAbstractTestLogger::outputString() would
crash, so add a QTEST_ASSERT to make the cause of any crashes more
obvious.

Change-Id: I00afe2e73120b87e211f858402d441f345dddd08
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>

src/testlib/qabstracttestlogger.cpp

@@ -93,6 +93,7 @@ void QAbstractTestLogger::filterUnprintable(char *str) const
 void QAbstractTestLogger::outputString(const char *msg)
 {
     QTEST_ASSERT(stream);
+    QTEST_ASSERT(msg);
 
     char *filtered = new char[strlen(msg) + 1];
     strcpy(filtered, msg);