RemoteLinux: Fix crash

When adding a Docker device via the Add Device Wizard,
while the docker executable is running on a remote linux device,
a crash would happen because the linux device is destroyed
due to a call to "saveSettings".

This patch adds a lock to the device shell mutex during destruction
to try and keep the device alive while a LinuxDevicePrivate::runInShell
call is still ongoing.

Fixes: QTCREATORBUG-31364
Change-Id: Ia154b527bb8344fc41fa91a56a7ef3305bfef0fa
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/remotelinux/linuxdevice.cpp

@@ -1170,6 +1170,7 @@ LinuxDevicePrivate::LinuxDevicePrivate(LinuxDevice *parent)
 
 LinuxDevicePrivate::~LinuxDevicePrivate()
 {
+    QMutexLocker locker(&m_shellMutex);
     auto closeShell = [this] {
         m_shellThread.quit();
         m_shellThread.wait();