Fix encode buffer in UCS-4
The current encoding buffer is 6 bytes. When the external format is :ucs-4 the first character needs a buffer of 8 because of the BOM.
Discovered in Clasp here: https://irclog.tymoon.eu/libera/%23clasp?around=1715274340#1715274340
Fix found here: https://irclog.tymoon.eu/libera/%23clasp?around=1715288971#1715288971