cid#1660144 silence bogus Data race condition

Change-Id: I023214c154ab3a44fca3b3b504c20b4670da2f99
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/190444
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Caolán McNamara <caolan.mcnamara@collabora.com>

extensions/source/update/check/updatecheck.cxx

@@ -934,6 +934,8 @@ UpdateCheck::shutdownThread(bool join)
 void
 UpdateCheck::enableAutoCheck(bool enable)
 {
+    // The (recursive) mutex is (at time of writing) always locked already here
+    std::scoped_lock aGuard(m_aMutex);
     if( enable )
     {
         m_updateCheckRunning = true;