Fix go vet errors (#473)

[ncopa]

Fixes go vet errors:

# github.com/sipcapture/heplify-server/rotator
rotator/rotator.go:222:2: unreachable code
# github.com/sipcapture/heplify-server/decoder
decoder/decoder.go:299:35: possible misuse of reflect.SliceHeader

see golang/go#40701

[CLAassistant]

All committers have signed the CLA.

[adubovikov]

Thank you!. Can you please sign CLA ?

[ncopa]

please have a close look at those changes, because

• I have no clue what this code actually does
• The rotator thing may return an error where it previously ignored it.
• I'm mainly doing a qualified guess that the reflect.SliceHeader fix is correct based on what I saw other projects do with the same error - but I don't really understand what happens.

[lmangani]

Changes approved, minor version increase required for a new package to be built