DF-3988 — dc_link_dp_set_test_pattern dereferences pipes[i].stream->sink
without a NULL check (dc_link_dp.c:2606): pipe_ctx[].stream is NULL for
every inactive pipe, and sink-initiated automated-test requests arrive
without any active stream on the tested link → NULL-pointer deref panic.
Also :2653 blanks `pipes[0]`'s stream_enc instead of the matched pipe_ctx
(wrong object; second NULL deref when pipe0 is an inactive hole).

BUILD
    cc -O2 -Wall -Wextra -o trigger trigger.c

RUN
    ./trigger      (model of the :2605-2610 scan: pipes[0]=NULL hole,
                    match at pipes[1] → NULL DEREF on i=0)

On the audit guest: builds and runs clean (run.log). Hardware-gated: guest
has bochs VGA only, zero 0x1002 PCI devices, amdgpu.ko not loaded.

EXPECTED
    "i=0 pipes[0].stream=0x0 -> ...->sink->link: NULL DEREF (panic)"
    and the :2653 wrong-object note; TRIGGER_RC=0.