smb/client: avoid dereferencing rdata=NULL in smb2_new_read_req()

This happens when called from SMB2_read() while using rdma
and reaching the rdma_readwrite_threshold.

Cc: [e-mail]
Fixes: a6559cc1d35d ("cifs: split out smb3_use_rdma_offload() helper")
Reviewed-by: David Howells <[e-mail]>
Signed-off-by: Stefan Metzmacher <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/smb/client/smb2pdu.c

···
 	 * If we want to do a RDMA write, fill in and append
 	 * smbd_buffer_descriptor_v1 to the end of read request
 	 */
-	if (smb3_use_rdma_offload(io_parms)) {
+	if (rdata && smb3_use_rdma_offload(io_parms)) {
 		struct smbd_buffer_descriptor_v1 *v1;
 		bool need_invalidate = server->dialect == SMB30_PROT_ID;