Fix multiple Coverity warnings in tftpd(8)
- Initialize uninitialized variable (CID 1006502)
- Delete dead code (CID 1006796, 1006794, 1006793)
- strcpy => strlcpy (CID 1006792, 1006791, 1006790)
- Check function return values (CID 1009442, 1009441, 1009440)
CID: 1006502, 1006796, 1006794, 1006793, 1006792, 1006791, 1006790,
CID: 1009442, 1009441, 1009440