hw/audio/virtio-snd: Remove unused assignment

hw/audio/virtio-snd: Remove unused assignment

Coverity reported:

  >>>     CID 1542933:  Code maintainability issues  (UNUSED_VALUE)
  >>>     CID 1542934:  Code maintainability issues  (UNUSED_VALUE)
  >>>     Assigning value "NULL" to "stream" here, but that stored
          value is overwritten before it can be used.

Simply remove the unused assignments.

Resolves: Coverity CID 1542933
Resolves: Coverity CID 1542934
Fixes: 731655f87f ("virtio-snd: rewrite invalid tx/rx message handling")
Fixes: 20cd0c8655 ("virtio-snd: rewrite invalid tx/rx message handling" in stable-8.2)
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-Id: <20240410053712.34747-1-philmd@linaro.org>
(cherry picked from commit dcb0a1ac03d6b5ba6c7fcbe467f0215738006113)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>