support set of non terminated string (with len)

lib/capn.c

@@ -1053,6 +1053,7 @@ capn_ptr capn_new_string(struct capn_segment *seg, const char *str, ssize_t sz)
 	new_object(&p, p.len);
 	if (p.data) {
 		memcpy(p.data, str, p.len - 1);
+		p.data[p.len - 1] = '\0';
 	}
 	return p;
 }