Merge r258758 (by adrian) from head:

Make sure any waiters on the scan results get notified if the scan task
  decides to do nothing.

  If this isn't done, then a scan request whilst a scan occurs in an
  active channel set or a completed channel set will hang.

Candidate for 10.0.

sys/net80211/ieee80211_scan.c

@@ -859,6 +859,7 @@ scan_task(void *arg, int pending)
 	if (ss->ss_next == ss->ss_last) {
 		IEEE80211_DPRINTF(vap, IEEE80211_MSG_SCAN,
 			"%s: no channels to scan\n", __func__);
+		scandone = 1;
 		goto done;
 	}