Add assertions to libevent callbacks.

They should never be called with a UTP socket.

libtransmission/peer-io.c

@@ -218,6 +218,7 @@ event_read_cb( int fd, short event UNUSED, void * vio )
     const unsigned int max = 256 * 1024;
 
     assert( tr_isPeerIo( io ) );
+    assert( io->socket >= 0 );
 
     io->pendingEvents &= ~EV_READ;
 
@@ -293,6 +294,7 @@ event_write_cb( int fd, short event UNUSED, void * vio )
     const tr_direction dir = TR_UP;
 
     assert( tr_isPeerIo( io ) );
+    assert( io->socket >= 0 );
 
     io->pendingEvents &= ~EV_WRITE;