Can't handle optional PBKDF2-params's keyLength

[emmansun]

When i tried to call KEYUTIL.getKeyFromEncryptedPKCS8PEM, it's failed due to there are keyLength in the PBKDF2-params. Per check the implementation, found the code get prf from position 2 directly:

jsrsasign/src/keyutil-1.0.js

Line 829 in 58bb241

var prf = aryval(pASNKDF, "2.seq.0.oid", "hmacWithSHA1");