bgpd: fix last_reset_cause setup

bgpd: fix last_reset_cause setup

last_reset_cause_size is the length *used* in last_reset_cause[].  It's
straight up used wrong here; we're saving off a reset cause and need to
check against the *available* size in last_reset_cause[].

This could actually have led to (hopefully rare) crashes in the assert
there, since the assert condition might fail incorrectly.

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