lib: cli: don't run off graph "pre-end"

lib: cli: don't run off graph "pre-end"

Behind END_TKN, there is another graph node whose data pointer is
actually struct cmd_element instead of struct cmd_token.  Don't try to
interpret that as cmd_token.  This causes very interesting crashes when
ASLR decides to give one of the strings of a command definition a lower
32-bit value that is a valid cmd_token_type (e.g. FORK_TKN).

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>