The `switch (n_rear)` supports up to 5 rear channels, but our channel
set only had space for 3. Size the set properly to fix this.
This didn't actually cause any memory unsafety as `PUSH_CHAN` would stop
incrementing `n_rear` if the channel set is already full.
Thanks to @alatiera for noticing this.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4740>
DEF_CHANSET (front, 7);
DEF_CHANSET (side, 2);
DEF_CHANSET (front, 7);
DEF_CHANSET (side, 2);
DEF_CHANSET (lfe, 2);
DEF_CHANSET (top_front, 3);
DEF_CHANSET (top_center, 3);
DEF_CHANSET (lfe, 2);
DEF_CHANSET (top_front, 3);
DEF_CHANSET (top_center, 3);