Don't compare invalid iterators in wxCommandProcessor code.

Don't compare invalid iterators in wxCommandProcessor code.

Comparing invalid iterators results in an assert failure in STL build.

We might also change wxList::compatibility_iterator::operator==() to allow
comparing invalid iterators (which should be different from all the other ones
but what about comparing two invalid iterators?) but it would probably be
better to get rid of all uses of compatibility_iterator in the code instead in
the long term.

Closes #12730.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66287 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775