fix suggest piece bug

src/peer_connection.cpp

@@ -3002,7 +3002,10 @@ namespace libtorrent
 			t->get_suggested_pieces(ret);
 			for (std::vector<int>::iterator i = ret.begin()
 				, end(ret.end()); i != end; ++i)
+			{
+				TORRENT_ASSERT(*i >= 0);
 				send_suggest(*i);
+			}
 
 			m_sent_suggests = true;
 		}

src/torrent.cpp

@@ -6825,6 +6825,8 @@ namespace libtorrent
 		// if we have an explicit cache, it's much more likely to
 		// stick around, so we should suggest all pieces
 		int num_pieces_to_suggest = int(ret.size());
+		if (num_pieces_to_suggest == 0) return;
+
 		if (!settings().explicit_read_cache)
 			num_pieces_to_suggest = (std::max)(1, int(ret.size() / 2));
 		ret.resize(num_pieces_to_suggest);