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.

uniq.c

@@ -76,7 +76,7 @@ uniq(FILE *fp, FILE *ofp)
 	size_t size = 0;
 	ssize_t len;
 
-	while ((len = getline(&buf, &size, fp)) >= 0)
+	while ((len = getline(&buf, &size, fp)) > 0)
 		uniqline(ofp, buf, (size_t)len);
 }