libmp3lame is compiled with assert() enabled

When encoding to mp3, VLCKit may be terminated due to an assert in L3psycho_anal_vbr.

This might be due to an actual problem. However, generally, libmp3lame should not be compiled with assert() enabled in production builds.

See https://rink.hockeyapp.net/manage/apps/194470/app_versions/122/crash_reasons/263164801