Decode AAC return error when AU data length is zero

[BenLocal]

try fix https://github.com/bluenviron/gortsplib/issues/542

[aler9]

Hello, although a blank access unit is surely an invalid unit, the RTP decoder is not in charge of validating incoming access units. Checks inside the RTP decoder are meant to avoid panics and OOM situations. Adding additional checks would decrease performance and would not be enough anyway to guarantee the validity of the unit.

Checks on the content of access units are already performed by codec-specific decoders or interpreters, during decoding or parsing.