fix mis-sizing of string in jbuf implementation

Any time the jbuf string grows, the head of the jbuf must be accounted for.
This patch fixes that oversight. The oversight leads to writing past
the end of the string after it is expanded.

lib/src/ovis_json/ovis_json.c

@@ -62,10 +62,10 @@ jbuf_t jbuf_append_va(jbuf_t jb, const char *fmt, va_list _ap)
 	cnt = vsnprintf(&jb->buf[jb->cursor], space, fmt, ap);
 	va_end(ap);
 	if (cnt >= space) {
-		space = jb->buf_len + cnt + JSON_BUF_START_LEN;
+		space = jb->buf_len + cnt + JSON_BUF_START_LEN + sizeof(*jb);
 		jb = realloc(jb, space);
 		if (jb) {
-			jb->buf_len = space;
+			jb->buf_len = space - sizeof(*jb);
 			goto retry;
 		} else {
 			return NULL;