While the nr_channels is defined with 3 bits, which allows up to 7 channels, actually the number of channels is less or equal to 4. This adds an assertion that helps static analyzers to avoid several false positives related with this. Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com> Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32589>