Correct UTF-8 encoding of U+FFFF

Overlong (and hence invalid) 4-byte encoding was used for this character
instead of the correct 3-byte 0xEF 0xBF 0xBF sequence.

Fix this by using 3 bytes for the code points up to 0xFFFF included,
instead of excluding it as was done before.

Closes #17920.

src/common/ustring.cpp

@@ -431,7 +431,7 @@ wxScopedCharBuffer wxUString::utf8_str() const
         {
             utf8_length += 2;
         }
-        else if ( code < 0xFFFF )
+        else if ( code <= 0xFFFF )
         {
             utf8_length += 3;
         }