Initialize the number of items in wxSelectionStore.

wxSelectionStore::m_count was somehow never initialized, and even although
this seems harmless right now, don't leave it uninitialized if only to avoid
Valgrind warnings about using this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75094 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775