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.
diff --git a/sort.c b/sort.c
index 00061fd..cb6f851 100644
--- a/sort.c
+++ b/sort.c
@@ -62,8 +62,9 @@
 	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;