merged fix RC_0_16

ChangeLog

@@ -5,6 +5,7 @@
 	* fix uTP edge case where udp socket buffer fills up
 	* fix nagle implementation in uTP
 
+	* fix potential crash when using torrent_handle::add_piece
 	* added missing add_torrent_alert to python binding
 
 0.16.5 release

src/torrent.cpp

@@ -3103,6 +3103,11 @@ namespace libtorrent
 
 		TORRENT_ASSERT(valid_metadata());
 
+		// it's possible to get here if the last piece was downloaded
+		// from peers and inserted with add_piece at the same time.
+		// if we're a seed, we won't have a piece picker, and can't continue
+		if (is_seed()) return;
+
 		TORRENT_ASSERT(!m_picker->have_piece(index));
 
 		state_updated();