-Werror,-Wuninitialized

(in SvxGraphCtrlAccessibleContext ctor: base class 'comphelper::OBaseMutex' is
uninitialized when used here to access 'comphelper::OBaseMutex::m_aMutex')

Change-Id: Ic6791043828b7b9f69c76867e26ee02a5ee66817