Don't pass logs at unknown levels to wxLog::DoLogRecord() from wxLogGui.

Don't pass logs at unknown levels to wxLog::DoLogRecord() from wxLogGui.

This results in an assert in DoLogText() which is not implemented in the base
class but ends up being called as DoLogTextAtLevel() doesn't know how to
handle non-standard log levels otherwise. This assert happened if you simply
called wxLogMessage(wxLOG_User, ...) in the program.

Just ignore messages at unknown log levels instead in wxLogGui, by definition
it can't handle them anyhow.

See also r63167.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63275 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775