Don't compare invalid iterators in wxCommandProcessor::IsDirty().

Don't compare invalid iterators in wxCommandProcessor::IsDirty().

Check that m_lastSavedCommand is valid before comparing it with
m_currentCommand as comparing invalid iterators results in an assert in STL
build (and probably should also result in an assert in non-STL build too for
consistency).

Also move IsDirty() implementation out of line to make it easier to modify it
further in the future and because there is no real reason to keep it inline.

Closes #13465.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70459 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775