rxrpc: Fix rxrpc_recvmsg tracepoint

Fix the rxrpc_recvmsg tracepoint to handle being called with a NULL call
parameter.

Fixes: a25e21f0 ("rxrpc, afs: Use debug_ids rather than pointers in traces")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/trace/events/rxrpc.h

@@ -1068,7 +1068,7 @@ TRACE_EVENT(rxrpc_recvmsg,
 			     ),
 
 	    TP_fast_assign(
-		    __entry->call = call->debug_id;
+		    __entry->call = call ? call->debug_id : 0;
 		    __entry->why = why;
 		    __entry->seq = seq;
 		    __entry->offset = offset;