Revalidate the adbname when canceling the ADB find

When canceling the ADB find, the lock on the find gets released for
a brief period of time to be locked again inside adbname lock.  During
the brief period that the ADB find is unlocked, it can get canceled by
other means removing it from the adbname list which in turn causes
assertion failure due to a double removal from the adbname list.

Recheck if the find->adbname is still valid after acquiring the lock
again and if not just skip the double removal.  Additionally, attach to
the adbname as in the worst case, the adbname might also cease to exist
if the scheduler would block this particular thread for a longer period
of time invalidating the lock we are going to acquire and release.

lib/dns/adb.c

@@ -2314,17 +2314,29 @@ dns_adb_cancelfind(dns_adbfind_t *find) {
 		 * locks in that order, to match locking hierarchy
 		 * elsewhere.
 		 */
+		dns_adbname_ref(adbname);
 		UNLOCK(&find->lock);
+
+		/*
+		 * Other thread could cancel the find between the unlock and
+		 * lock, so we need to recheck whether the adbname is still
+		 * valid and reference the adbname, so it does not vanish before
+		 * we have a chance to lock it again.
+		 */
+
 		LOCK(&adbname->lock);
 		LOCK(&find->lock);
 
-		ISC_LIST_UNLINK(adbname->finds, find, plink);
-		find->adbname = NULL;
+		if (find->adbname != NULL) {
+			ISC_LIST_UNLINK(find->adbname->finds, find, plink);
+			find->adbname = NULL;
+		}
 
 		find_sendevent(find);
 
 		UNLOCK(&find->lock);
 		UNLOCK(&adbname->lock);
+		dns_adbname_detach(adbname);
 	}
 }