drm/dp_mst: fix multiple frees of tx->bytes

Currently tx->bytes is being freed r->num_transactions number of
times because tx is not being set correctly. Fix this by setting
tx to &r->transactions[i] so that the correct objects are being
freed on each loop iteration.

Addresses-Coverity: ("Double free")
Fixes: 2f015ec6 ("drm/dp_mst: Add sideband down request tracing + selftests")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120173509.347490-1-colin.king@canonical.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -504,8 +504,10 @@ drm_dp_decode_sideband_req(const struct drm_dp_sideband_msg_tx *raw,
 			}
 
 			if (failed) {
-				for (i = 0; i < r->num_transactions; i++)
+				for (i = 0; i < r->num_transactions; i++) {
+					tx = &r->transactions[i];
 					kfree(tx->bytes);
+				}
 				return -ENOMEM;
 			}