RFR: 8262329: Fix JFR parser exception messages

[Roman Kennke]

I've been irritated by unreasonable large constant pool offsets in ChunkParser excepion messages. It turns out that lastCP + size concatenates the two numbers as strings instead of adding them.

Testing:
 - [x] manually inspecting the exception messages

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Commit messages:
 - 8262329: Fix JFR parser exception messages

Changes: https://git.openjdk.java.net/jdk/pull/2714/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2714&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262329
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2714.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2714/head:pull/2714

PR: https://git.openjdk.java.net/jdk/pull/2714