aac: prevent crash on malformed AU headers - #2513

Summary

RTPToADTS trusted MPEG4-GENERIC AU-header lengths before slicing the RTP payload. A malformed or truncated AAC RTP packet could therefore cause an out-of-bounds slice panic and terminate the entire go2rtc process.

This change validates:

• the minimum RTP payload size;
• AU-header length and alignment;
• each declared AU size against the remaining payload.

Invalid packets are dropped instead of being forwarded or crashing go2rtc.

Also updates two stale AAC test expectations to use the current uint8 channel-count type.

Testing

go test ./pkg/aac

Related

Similar malformed-AAC handling was previously addressed in the RTP depacketizer path in #697; this covers the remaining RTPToADTS path.