Fixed a use after free that occured when doing DHT_Disconnect while the client was...

Fixed a use after free that occured when doing DHT_Disconnect while the client was trying to reconnect to the service.

==12088== Invalid read of size 8
==12088==    at 0x5245EB4: GNUNET_CLIENT_notify_transmit_ready_cancel (client.c:1118)
==12088==    by 0x5033F10: GNUNET_DHT_disconnect (dht_api.c:571)
==12088==    by 0x403211: shutdown_task (gnunet-service-mesh.c:3366)
==12088==    by 0x5260292: GNUNET_SCHEDULER_run (scheduler.c:682)
==12088==    by 0x526634B: GNUNET_SERVICE_run (service.c:1590)
==12088==    by 0x401885: main (gnunet-service-mesh.c:3460)
==12088==  Address 0x651fb90 is 32 bytes inside a block of size 64 free'd
==12088==    at 0x4C2556E: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==12088==    by 0x5245F85: GNUNET_CLIENT_disconnect (client.c:422)
==12088==    by 0x503360A: do_disconnect (dht_api.c:323)
==12088==    by 0x5260292: GNUNET_SCHEDULER_run (scheduler.c:682)
==12088==    by 0x526634B: GNUNET_SERVICE_run (service.c:1590)
==12088==    by 0x401885: main (gnunet-service-mesh.c:3460)