Don't assert if config file contains an invalid boolean value.

Don't assert if config file contains an invalid boolean value.

Asserts should be only triggered by programming errors, not by user actions,
and the assert checking that the value is either 0 or 1 in
wxConfigBase::DoReadBool() could happen if the user edited the file and put a
wrong value into it.

Replace the assert with a warning message.

See #11437.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66140 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775