Be more permissive when validating format string arguments.

Previously, the code would assert if the caller passed too many
arguments to wxPrintf() or other printf-like functions. But that can
happen legitimately in translations: in some languages such as Hebrew,
using "1" (i.e. "%d") in the singular feels unnatural and it's better to
use the word "one" and left the variadic argument unused.

Relax the check not to assert in this case. This is consistent with the
standard library and other implementations. Notice that gettext's msgfmt
doesn't complain about this case either in the specific case of singular
forms.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76027 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775