RFR 8147502: Digest is incorrectly truncated for ECDSA signatures...

[Adam Petcher]

webrev: https://cr.openjdk.java.net/~apetcher/8147502/webrev.00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8147502

Please review this fix to a bug that causes ECDSA signatures to be 
incorrect in some cases. The fix is simple, but testing this issue is 
difficult because the API doesn't give access to the raw signing 
operation so we can check it using known answer tests. I got around this 
difficulty in the regression test by using a modified SecureRandom that 
supplies specific bits in order to produce the correct nonce. The test 
is a bit complicated and brittle, so if anyone has any other suggestions 
on how to do this, please share.