utmpdump: check ftello() return value

Signed-off-by: Karel Zak <kzak@redhat.com>

login-utils/utmpdump.c

@@ -169,6 +169,9 @@ static int follow_by_inotify(FILE *in, const char *filename, FILE *out)
 	size = ftello(in);
 	fclose(in);
 
+	if (size < 0)
+		err(EXIT_FAILURE, _("%s: cannot get file position"), filename);
+
 	wd = inotify_add_watch(fd, filename, EVENTS);
 	if (wd == -1)
 		err(EXIT_FAILURE, _("%s: cannot add inotify watch."), filename);