Fix qErrnoWarning() printing no error in some conditions

Because it allocates memory, it may call system functions that set errno
or Win32's GetLastError().

[ChangeLog][QtCore] Fixed a bug that made qErrnoWarning() say there was
no error when generating the error message.

Fixes: QTBUG-77322
Change-Id: Ife213d861bb14c1787e1fffd15b811a4f83cf3e7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/global/qlogging.cpp

@@ -1923,12 +1923,14 @@ void qErrnoWarning(const char *msg, ...)
 {
     // qt_error_string() will allocate anyway, so we don't have
     // to be careful here (like we do in plain qWarning())
+    QString error_string = qt_error_string(-1);  // before vasprintf changes errno/GetLastError()
+
     va_list ap;
     va_start(ap, msg);
     QString buf = QString::vasprintf(msg, ap);
     va_end(ap);
 
-    buf += QLatin1String(" (") + qt_error_string(-1) + QLatin1Char(')');
+    buf += QLatin1String(" (") + error_string + QLatin1Char(')');
     QMessageLogContext context;
     qt_message_output(QtCriticalMsg, context, buf);
 }