ipsec esp: avoid dereferencing freed secasindex

It is possible that SA was removed while processing packed, in which
case it is changed to the DEAD state and it index is removed from the
tree. Dereferencing sav->sah then touches freed memory.

Reviewed by: ae
Sponsored by: NVIDIA networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44079