MFC r336609:

MFC r336609:

Fix several Coverity warnings in tftp

Some of the changes are in the libexec/tftpd directory, but to functions that
are only used by tftp(1) (they share some code).

* strcpy => strlcpy (1006793, 1006794, 1006796, 1006741)
* Unchecked return value and TOCTTOU (1009314)
* NULL pointer dereference (1018035, 1018036)

Reported by:	Coverity
CID:		1006793, 1006794, 1006796, 1006741, 1009314, 1018035
CID:		1018036

git-svn-id: svn://svn.freebsd.org/base/stable/10@339060 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f