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: stable@vger.kernel.org
Fixes: a6559cc1 ("cifs: split out smb3_use_rdma_offload() helper")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2pdu.c

@@ -4441,7 +4441,7 @@ smb2_new_read_req(void **buf, unsigned int *total_len,
 	 * 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;