SUNRPC: Fix general protection fault in trace_rpc_xdr_overflow()

The TP_fast_assign() section is careful enough not to dereference
xdr->rqst if it's NULL. The TP_STRUCT__entry section is not.

Fixes: 5582863f ("SUNRPC: Add XDR overflow trace event")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/trace/events/sunrpc.h

@@ -655,10 +655,10 @@ TRACE_EVENT(rpc_xdr_overflow,
 		__field(size_t, tail_len)
 		__field(unsigned int, page_len)
 		__field(unsigned int, len)
-		__string(progname,
-			 xdr->rqst->rq_task->tk_client->cl_program->name)
-		__string(procedure,
-			 xdr->rqst->rq_task->tk_msg.rpc_proc->p_name)
+		__string(progname, xdr->rqst ?
+			 xdr->rqst->rq_task->tk_client->cl_program->name : "unknown")
+		__string(procedure, xdr->rqst ?
+			 xdr->rqst->rq_task->tk_msg.rpc_proc->p_name : "unknown")
 	),
 
 	TP_fast_assign(