fix memory leak of pointers contained in m_aErrDescList

Change-Id: I9fcbdcd54978ccaffa5359c0afb0a1990356c218
Reviewed-on: https://gerrit.libreoffice.org/12205
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>