RFR: 8260403: javap should be more robust in the face of invalid class files

[Adam Sotona]

Please review javap fix to handle java.lang.IllegalStateException for classes with invalid Signature attribute.
New test (T8260403) parsing class with invalid Signature attribute (as described in the bug) is included.
Fix wraps java.lang.IllegalStateException, reports "Error: Invalid value for Signature attribute: <signature debug info>" and continues.

Thanks,
Adam

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Commit messages:
 - 8260403: javap should be more robust in the face of invalid class files

Changes: https://git.openjdk.java.net/jdk/pull/2724/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2724&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260403
  Stats: 165 lines in 3 files changed: 164 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2724.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2724/head:pull/2724

PR: https://git.openjdk.java.net/jdk/pull/2724