degesch: fix a minor bug in buffer_merge()

The pointer to the last item in the linked list wasn't always fixed,
although nothing really touched it afterwards.

degesch.c

@@ -4221,7 +4221,6 @@ buffer_merge (struct app_context *ctx,
 		merged->lines = NULL;
 	else if (start->prev)
 		start->prev->next = NULL;
-	if (start == merged->lines_tail)
 	merged->lines_tail = start->prev;
 	merged->lines_count -= n;