let _safeProtoExtend function safer.
let _safeProtoExtend function safer.

_safeProtoExtend function will reset a non-exist function to 'undefined', so it is not safe enough.
NetEaseWD/NEJ,jesseband/nej,NetEaseWD/NEJ,NEYouFan/nej-framework,JuneAndGreen/nej,jesseband/nej,JuneAndGreen/nej,NEYouFan/nej-framework