Add even more checks and guards to the music loading process to ensure
proper metadata loading.
Auxio has always had issues handling track numbers and years, mostly
with ensuring their validity. This change resolves them by more or less
surrounding them with gobs of null-checks and fallible parsing
routines, which should help avoid frustrating bugs and crashes with
metadata in the future.
Resolves#88.
Resolves#84.
1e39ceb9fc