have wxbuiltin wxmbconvutf7 return -1 (error) not 0. Some cleanups in string.cpp. Add some unit tests for utf7 and cleanups.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30162 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/strconv.cpp

@@ -248,14 +248,14 @@ size_t wxMBConvUTF7::MB2WC(wchar_t * WXUNUSED(buf),
                            const char * WXUNUSED(psz),
                            size_t WXUNUSED(n)) const
 {
-  return 0;
+  return (size_t)-1;
 }
 
 size_t wxMBConvUTF7::WC2MB(char * WXUNUSED(buf),
                            const wchar_t * WXUNUSED(psz),
                            size_t WXUNUSED(n)) const
 {
-  return 0;
+  return (size_t)-1;
 }
 
 // ----------------------------------------------------------------------------