RFR: 8287246: DSAKeyValue should check for missing params instead of relying on KeyFactory provider [v2]

[Sean Mullan]

> Please review this fix to the XML Signature implementation to check for null or missing DSA parameters and throw a MarshalException before trying to create a DSA public key from its XML encoding. This will allow the code to fail earlier and not depend on the underlying provider to detect illegal or missing parameters.

Sean Mullan has updated the pull request incrementally with one additional commit since the last revision:

  Clarify why we don't support missing DSA params.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8870/files
  - new: https://git.openjdk.java.net/jdk/pull/8870/files/e53e205c..d3d9ab00

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8870&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8870&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8870.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8870/head:pull/8870

PR: https://git.openjdk.java.net/jdk/pull/8870