detect: fix alstate handling

detect: fix alstate handling

Previously, the alstate use in the main detect loop was unsafe. The
alstate pointer would be set duing a lock, but it would again be used
after one or more lock/unlock cycles. If the data pointed to would
disappear, a dangling pointer would be the result.

Due to they way flows are cleaned up using reference counting and
such, changes of this happening were very small. However, at least
one path can lead to this situation. So it had to be fixed.