ipsec esp: avoid dereferencing freed secasindex

[kib]

Summary

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

[ae]

Probably we should increase esps_notdb or esps_invalid counter here.

[kib]

Update counter, add debug printf for the situation.