internal/maindata: suppress out-of-range error

Updates #51

fix: make initialization of nSlen2 safe for concurrent usage (#49)

The concurrent implementation of nSlen2 initialization causes a data race
if multiple mp3's are read at the same time and nSlen2 hasn't been already initialized
this fixes the race condition by initializing the nSlen2 once

Fix compile errors (#45)

Fix failure to build with Go 1.12

Replace example MPEG2 file (#36)

Replace the provided example MPEG2 file by a copyright free version (using the beginning of
Alice's Adventures in Wonderland by Lewis Carroll, published in 1865).

bump oto to v0.6.1 so gonic can build on {Open,Free}BSD (#34)

Update Oto version

Breaking change: Remove (*Decoder).Close

Add Sergei to AUTHORS

Update go.mod