When flushing packets from the powersave queue, make sure that

m_nextpkt is NULL before passing it up to the parent transmit
method.

sys/net80211/ieee80211_power.c

@@ -456,6 +456,7 @@ pwrsave_flushq(struct ieee80211_node *ni)
 		while (parent_q != NULL) {
 			m = parent_q;
 			parent_q = m->m_nextpkt;
+			m->m_nextpkt = NULL;
 			/* must be encapsulated */
 			KASSERT((m->m_flags & M_ENCAP),
 			    ("%s: parentq with non-M_ENCAP frame!\n",