if the input string contains invalid utf-8, runetochar() produces
unicode replacement characters that can overflow the literal buffer.
as the overflow check is done after runetochar(), add UTFmax bytes
of slack space avoiding the issue.
spew → I fixed the memory leak setjmp/longjmp problem with libjson
spew → http://www.spew.club/json.patch
spew → full file: http://www.spew.club/json.c
spew → going to bed, I'll annoy cinap_lenrek tomorrow to try to get this committed