Fix use of uninitialised value identified by valgrind

m_pLastSyncedDbusMenu was previously only initialised in one of
two constructors

Change-Id: Icd29d1ab314c54544b61ae60b57b29f1fa7d0e3d
Reviewed-on: https://gerrit.libreoffice.org/11530
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>