RFR 8054537: sun.security.x509.SerialNumber constructor should not accept negative numbers

[Svetlana Nikandrova]

Hello,

could you please review this simple fix.
Issue:
https://bugs.openjdk.java.net/browse/JDK-8054537
Webrev:
http://cr.openjdk.java.net/~snikandrova/8054537/webrev.00/ 
<http://cr.openjdk.java.net/%7Esnikandrova/8054537/webrev.00/>

Description:
Added check if SerialNumber constructor's parameter is negative.

Thank you,
Svetlana