Fix for BUG#7658 "optimize crashes slave thread (1 in 1000)]":

mysql_admin_table() attempted to write to a vio which was 0. I could have fixed mysql_admin_table()
but fixing my_net_write() looked more future-proof.

sql/net_serv.cc

@@ -227,6 +227,8 @@ int
 my_net_write(NET *net,const char *packet,ulong len)
 {
   uchar buff[NET_HEADER_SIZE];
+  if (unlikely(!net->vio)) // nowhere to write
+    return 0;
   /*
     Big packets are handled by splitting them in packets of MAX_PACKET_LENGTH
     length. The last packet is always a packet that is < MAX_PACKET_LENGTH.