properly NUL-terminate the output in wxMBConvUTF16swap::WC2MB()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38575 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/strconv.cpp

@@ -991,8 +991,11 @@ size_t wxMBConvUTF16swap::WC2MB(char *buf, const wchar_t *psz, size_t n) const
         psz++;
     }
 
-    if ( buf && len < n )
+    if ( buf && len < n - 1 )
-        *buf = '\0';
+    {
+        buf[0] =
+        buf[1] = '\0';
+    }
 
     return len;
 }