drm/selftests/test-drm_dp_mst_helper: Fix memory leak in sideband_msg_req_encode_decode

Avoid leaking the "out" variable if it is not possible to allocate
the "txmsg" variable.

Fixes: 09234b88 ("drm/selftests/test-drm_dp_mst_helper: Move 'sideband_msg_req_encode_decode' onto the heap")
Addresses-Coverity-ID: 1475685 ("Resource leak")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220108165812.46797-1-jose.exposito89@gmail.com

drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c

@@ -131,8 +131,10 @@ sideband_msg_req_encode_decode(struct drm_dp_sideband_msg_req_body *in)
 		return false;
 
 	txmsg = kzalloc(sizeof(*txmsg), GFP_KERNEL);
-	if (!txmsg)
+	if (!txmsg) {
+		kfree(out);
 		return false;
+	}
 
 	drm_dp_encode_sideband_req(in, txmsg);
 	ret = drm_dp_decode_sideband_req(txmsg, out);