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.

nl.c

@@ -50,7 +50,7 @@ nl(const char *fname, FILE *fp)
 	int donumber, oldsection, section = 1, bl = 1;
 	char *buf = NULL;
 
-	while (getline(&buf, &size, fp) >= 0) {
+	while (getline(&buf, &size, fp) > 0) {
 		donumber = 0;
 		oldsection = section;