New version without the hardcoded limit: don't use goto encodeUCS4;, chain if to limit indentation depth: it only costs one copy of the UCS4 (5 lines are duplicated).

The buffer is now reallocated each time a surrogate escape is longer than 4 bytes.

I don't know if "nallocated += repsize - 4;" can overflow or not. If yes, how can I detect the overflow? I added: /* FIXME: check integer overflow? */