Fix wx2stclen() return values after upgrade to Scintilla 3.5.5.

The fix to the length allocated for the buffer (which was off by 1) in
wx2stc() in 9f81ac16f0 exposed another bug,
which used to compensate for that one, in wx2stclen(), so fix it too.

See #16776.

include/wx/stc/private.h

@@ -28,7 +28,7 @@ extern wxCharBuffer wx2stc(const wxString& str);
 // just to compute the length.
 inline size_t wx2stclen(const wxString& WXUNUSED(str), const wxCharBuffer& buf)
 {
-    return buf.length() - 1;
+    return buf.length();
 }
 
 #else // not UNICODE