RFR: 8253299: Manifest bytes are read twice when verifying a signed JAR

[Hai-May Chao]

Small change to retrieve the raw bytes of manifest during verifying signed JAR.

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Commit messages:
 - 8253299: Manifest bytes are read twice when verifying a signed JAR

Changes: https://git.openjdk.java.net/jdk/pull/1299/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1299&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253299
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1299.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1299/head:pull/1299

PR: https://git.openjdk.java.net/jdk/pull/1299