Check getline()-return-values properly

It's not useful when 0 is returned anyway, so be sure that we have a
string with length > 0, this also solves some indexing-gotchas like
"len - 1" and so on.
Also, add checked getline()'s whenever it has been forgotten and
clean up the error-messages.

sort.c

@@ -63,9 +63,9 @@ check(FILE *fp)
 {
 	static struct { char *buf; size_t size; } prev, cur, tmp;
 
-	if (!prev.buf)
-		getline(&prev.buf, &prev.size, fp);
-	while (getline(&cur.buf, &cur.size, fp) != -1) {
+	if (!prev.buf && getline(&prev.buf, &prev.size, fp) < 0)
+		eprintf("getline:");
+	while (getline(&cur.buf, &cur.size, fp) > 0) {
 		if (uflag > linecmp((const char **) &cur.buf, (const char **) &prev.buf)) {
 			if (!Cflag)
 				weprintf("disorder: %s", cur.buf);