RFR: JDK-8315042 NPE in PKCS7.parseOldSignedData [v4]

[Mark Powers]

> https://bugs.openjdk.org/browse/JDK-8315042

Mark Powers has updated the pull request incrementally with one additional commit since the last revision:

  more comments from Weijun

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15844/files
  - new: https://git.openjdk.org/jdk/pull/15844/files/b481ec44..369c3356

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15844&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15844&range=02-03

  Stats: 23 lines in 1 file changed: 5 ins; 12 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15844.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15844/head:pull/15844

PR: https://git.openjdk.org/jdk/pull/15844