scte35, psi: guard bounds before trusting pointer/length fields

[ChrisJr404]

NewSCTE35 and NewPMT index into their input before checking it fits, so malformed data panics instead of returning an error.

Two failure modes, both found by fuzzing:

• scte35.NewSCTE35 on empty input panics in psi.PointerField, which reads data[0] before the length check downstream runs. Added a len(data) < 1 guard so it returns ErrInvalidSCTE35Length like the other short-input cases.
• psi.NewPMT panics with a slice-out-of-range when the section length field is larger than the buffer (and on empty input, again via PointerField). parseTables now checks the pointer field and each table length before slicing, mirroring the guards PmtAccumulatorDoneFunc already does in the same file, and returns ErrShortPayload.

Valid input decodes the same as before. Added TestNewSCTE35Truncated and TestNewPMTTruncated with the reproduced inputs (panic before, error after). go build/vet/test all pass.

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