Catch empty output list in bs1770gain (fix #1398)
Catch empty output list in bs1770gain (fix #1398)

This should catch the original crash that brought this all up.

Andypsamp/CODfinalJUNIT,lightwang1/beets,lightwang1/beets,asteven/beets,moodboom/beets,Andypsamp/CODfinalJUNIT,drm00/beets,randybias/beets,LordSputnik/beets,jcoady9/beets,swt30/beets,sadatay/beets,imsparsh/beets,Andypsamp/CODfinalJUNIT,SusannaMaria/beets,SusannaMaria/beets,pkess/beets,jackwilsdon/beets,drm00/beets,ruippeixotog/beets,Kraymer/beets,gabrielaraujof/beets,kareemallen/beets,parapente/beets,randybias/beets,LordSputnik/beets,artemutin/beets,ruippeixotog/beets,sadatay/beets,mosesfistos1/beetbox,ttsda/beets,xsteadfastx/beets,kareemallen/beets,drm00/beets,mried/beets,beetbox/beets,Andypsamp/CODfinalJUNIT,untitaker/beets,Freso/beets,xsteadfastx/beets,marcuskrahl/beets,lengtche/beets,shamangeorge/beets,parapente/beets,Freso/beets,YetAnotherNerd/beets,mosesfistos1/beetbox,YetAnotherNerd/beets,Freso/beets,shanemikel/beets,kareemallen/beets,diego-plan9/beets,Dishwishy/beets,jcoady9/beets,marcuskrahl/beets,jmwatte/beets,Andypsamp/CODjunit,randybias/beets,moodboom/beets,madmouser1/beets,untitaker/beets,jcoady9/beets,asteven/beets,marcuskrahl/beets,tima/beets,gabrielaraujof/beets,artemutin/beets,asteven/beets,artemutin/beets,shamangeorge/beets,PierreRust/beets,m-urban/beets,ruippeixotog/beets,diego-plan9/beets,sampsyo/beets,mathstuf/beets,imsparsh/beets,lengtche/beets,xsteadfastx/beets,randybias/beets,PierreRust/beets,beetbox/beets,madmouser1/beets,SusannaMaria/beets,moodboom/beets,kelvinhammond/beets,shanemikel/beets,swt30/beets,diego-plan9/beets,dfc/beets,imsparsh/beets,m-urban/beets,pkess/beets,mathstuf/beets,ibmibmibm/beets,marcuskrahl/beets,mosesfistos1/beetbox,MyTunesFreeMusic/privacy-policy,untitaker/beets,moodboom/beets,kelvinhammond/beets,shanemikel/beets,ibmibmibm/beets,dfc/beets,arabenjamin/beets,tima/beets,beetbox/beets,PierreRust/beets,ibmibmibm/beets,ruippeixotog/beets,lightwang1/beets,Andypsamp/CODjunit,ttsda/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,lengtche/beets,parapente/beets,m-urban/beets,sampsyo/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,swt30/beets,sampsyo/beets,Andypsamp/CODjunit,sampsyo/beets,Dishwishy/beets,beetbox/beets,jmwatte/beets,mried/beets,dfc/beets,YetAnotherNerd/beets,jcoady9/beets,diego-plan9/beets,shamangeorge/beets,drm00/beets,gabrielaraujof/beets,arabenjamin/beets,Andypsamp/CODjunit,mosesfistos1/beetbox,lengtche/beets,madmouser1/beets,LordSputnik/beets,ttsda/beets,Andypsamp/CODfinalJUNIT,Dishwishy/beets,mathstuf/beets,mried/beets,pkess/beets,jmwatte/beets,m-urban/beets,jackwilsdon/beets,parapente/beets,swt30/beets,shanemikel/beets,tima/beets,kelvinhammond/beets,jmwatte/beets,pkess/beets,tima/beets,kelvinhammond/beets,dfc/beets,Freso/beets,jackwilsdon/beets,MyTunesFreeMusic/privacy-policy,lightwang1/beets,arabenjamin/beets,kareemallen/beets,xsteadfastx/beets,SusannaMaria/beets,shamangeorge/beets,sadatay/beets,arabenjamin/beets,YetAnotherNerd/beets,gabrielaraujof/beets,Andypsamp/CODjunit,imsparsh/beets,Kraymer/beets,PierreRust/beets,untitaker/beets,ttsda/beets,artemutin/beets,Kraymer/beets,LordSputnik/beets,asteven/beets,Dishwishy/beets,sadatay/beets,madmouser1/beets,jackwilsdon/beets,mried/beets,mathstuf/beets