leveldb: Provides clarity what actually trigger 'fatal error: fault'
leveldb: Provides clarity what actually trigger 'fatal error: fault'

Saves value returned by t.file.Num() to a variable before use it to
access the map. Also use bool as map element type instead of empty struct.

Related to issue #27.

no2key/goleveldb,syndtr/goleveldb,rentongzhang/goleveldb,zzl0/goleveldb,btcsuite/goleveldb,trigrass2/goleveldb,ngaut/goleveldb,hustbill/goleveldb,chai2010/goleveldb,conseweb/goleveldb,FactomProject/goleveldb