Actually move past the field separator

Previously, sort(1) failed on key-based sorting and was caught in an
infinite loop with the c-flag.

sort.c

@@ -62,8 +62,9 @@ skipcolumn(struct linebufline *a, int skip_to_next_col)
 	if (fieldsep) {
 		if ((s = memmem(a->data, a->len, fieldsep, fieldseplen))) {
 			if (skip_to_next_col) {
-				a->len = a->len - (s - a->data);
+				s += fieldseplen;
 				a->data = s;
+				a->len = a->len - (s - a->data);
 			}
 		} else {
 			a->data += a->len - 1;