libjson: add slack space to literal string buffer to handle bad runes (thanks mischief)

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.

sys/src/libjson/json.c

@@ -323,7 +323,7 @@ jsonparse(char *s)
 	memset(&l, 0, sizeof(l));
 	l.s = s;
 	l.slen = strlen(s);
-	if((l.buf = mallocz(l.slen+1, 1)) == nil)
+	if((l.buf = mallocz(l.slen+UTFmax+1, 1)) == nil)
 		return nil;
 
 	j = jsonobj(&l);
@@ -336,6 +336,8 @@ jsonfree(JSON *j)
 {
 	JSONEl *e, *f;
 
+	if(j == nil)
+		return;
 	switch(j->t){
 	case JSONString:
 		if(j->s)