Don't access sys_errlist[] directly (breaks uClibc 0.9.26)

diff -u -p -u -p -r1.4 tftpd.c
--- user/tftpd/tftpd.c	6 Sep 2003 00:51:08 -0000	1.4
+++ user/tftpd/tftpd.c	29 May 2004 14:23:51 -0000
@@ -480,7 +480,7 @@ nak(error)
 		if (pe->e_code == error)
 			break;
 	if (pe->e_code < 0) {
-		pe->e_msg = sys_errlist[error - 100];
+		pe->e_msg = strerror(error - 100);
 		tp->th_code = EUNDEF;   /* set 'undef' errorcode */
 	}
 	strcpy(tp->th_msg, pe->e_msg);