drm/mst: Avoid dereferencing a NULL mstb in drm_dp_mst_handle_up_req()

drm/mst: Avoid dereferencing a NULL mstb in drm_dp_mst_handle_up_req()

commit 7f8b3987da54cb4d41ad2545cd4d7958b9a36bdf upstream.

In case of an unknown broadcast message is sent mstb will remain unset,
so check for this.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Lyude <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719114330.26540-3-imre.deak@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>