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/strconv.cpp

@@ -1135,7 +1135,7 @@ wxMBConvStrictUTF8::FromWChar(char *dst, size_t dstLen,
                 out[0] = 0xC0 | code;
             }
         }
-        else if ( code < 0xFFFF )
+        else if ( code <= 0xFFFF )
         {
             len = 3;
             if ( out )