reverted the change of r56246 and keep the old behaviour of MB2WC/WC2MB; document it even more clearly and correct code using these functions incorrectly

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@56394 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/textctrl.cpp

@@ -947,14 +947,14 @@ wxTextCtrl::StreamIn(const wxString& value,
         return false;
 
 #if wxUSE_WCHAR_T
-    wxWCharBuffer wchBuf(len);
+    wxWCharBuffer wchBuf(len); // allocates one extra character
     wchar_t *wpc = wchBuf.data();
 #else
     wchar_t *wchBuf = (wchar_t *)malloc((len + 1)*sizeof(wchar_t));
     wchar_t *wpc = wchBuf;
 #endif
 
-    conv.MB2WC(wpc, value, value.length());
+    conv.MB2WC(wpc, value, len + 1);
 #endif // wxUSE_UNICODE_MSLU
 
     // finally, stream it in the control