Fix bootstrap applications crashing when QT_MESSAGE_PATTERN contains %{backtrace}

tokens[i] was left uninitialized. This could cause a crash when moc produces a
qWarning.

Task-number: QTBUG-56045
Change-Id: I6fba933005edd29756f0d6b1cfe53243254ac7b2
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qlogging.cpp

@@ -1133,6 +1133,7 @@ void QMessagePattern::setPattern(const QString &pattern)
                 backtraceArgs.append(backtraceParams);
 #else
                 error += QStringLiteral("QT_MESSAGE_PATTERN: %{backtrace} is not supported by this Qt build\n");
+                tokens[i] = "";
 #endif
             }