rxrpc: Fix a missing NULL-pointer check in a trace

Fix the rxrpc_client tracepoint to not dereference conn to get the cid if
conn is NULL, as it does for other fields.

	RIP: 0010:trace_event_raw_event_rxrpc_client+0x7e/0xe0 [rxrpc]
	Call Trace:
	 rxrpc_activate_channels+0x62/0xb0 [rxrpc]
	 rxrpc_connect_call+0x481/0x650 [rxrpc]
	 ? wake_up_q+0xa0/0xa0
	 ? rxrpc_kernel_begin_call+0x12a/0x1b0 [rxrpc]
	 rxrpc_new_client_call+0x2a5/0x5e0 [rxrpc]

Fixes: 245500d8 ("rxrpc: Rewrite the client connection manager")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>