Remove or reset the corresponding bitmap if the provided one is invalid instead of asserting, this is what wxGTK does and this behaviour seems to be more useful. Also document this behaviour as it's now implemented in both wxMSW and wxGTK. Closes #13569. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76404 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775