fixed another bug in strconv

fixed another bug in strconv
(Off by one error that did not demonstrate itself in poEdit before because
windows native convertion wasn't used due to wxLocale returning cp12xx instead
of windows-12xx. MultiByteToWideChar and WideCharToMultiByte return size of
needed buffer, not number of output characters. Author of the code didn't know
this, hence off by one.)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@10557 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775