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

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

Cc: Dave Airlie <[email protected]>
Cc: Lyude <[email protected]>
Cc: Daniel Vetter <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Lyude <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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