Check wxDateTime components validity more rigorously.

Check wxDateTime components validity more rigorously.

Check that the provided day is strictly positive and also that the month is in
valid range: while it should always be, considering that it's an enum element,
in practice people often cast ints to wxDateTime::Month with potentially fatal
results. Catch this with an assert in wxDateTime::Tm::IsValid().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66203 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775