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.

fold.c

@@ -55,7 +55,7 @@ fold(FILE *fp, const char *fname)
 	char *buf = NULL;
 	size_t size = 0;
 
-	while (getline(&buf, &size, fp) >= 0)
+	while (getline(&buf, &size, fp) > 0)
 		foldline(buf);
 	if (ferror(fp))
 		eprintf("getline %s:", fname);