RFR (XS) 8209851 : Algorithm name is compared via reference identity

[Ivan Gerasimov]

Hello!

In sun.security.provider.DSA.engineInitSign() a check for the key size 
is meant to be skipped if the MessageDigest is NullDigest20.

However, the check for the algorithm's name is done via comparing with a 
String literal, which is not guaranteed to be accurate.

Would you please help review the trivial fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8209851
WEBREV: http://cr.openjdk.java.net/~igerasim/8209851/00/webrev/

-- 
With kind regards,
Ivan Gerasimov