pflow: handle unattached states

It's possible for states to be cleaned up (through pf_detach_state()) that
have not been fully attached. For example if there's an ID conflict during
pf_state_insert().

pflow exports states from pf_detach_state(), so it can get called on such
states, but did not account for this and could end up dereferencing a NULL
state key.

Check for this in export_pflow() and do not export unattached states.

See also: https://redmine.pfsense.org/issues/15446
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