Set shut down flag after acquiring the mutex, not before

As this flag is tested in the worker thread only when it owns the mutex,
set it only after acquiring the mutex too to avoid any possibility of a
data race.

src/common/webrequest_curl.cpp

@@ -356,8 +356,8 @@ wxWebSessionCURL::~wxWebSessionCURL()
 {
     {
         // Notify the work thread
-        m_shuttingDown = true;
         wxMutexLocker lock(m_mutex);
+        m_shuttingDown = true;
         m_condition.Signal();
     }