RFR: 7337: JMC fails to parse JFR with events from WebLogic Server

[Guru Hb]

RC : NPE due to Invalid packageFieldIndex. 
Disassembled the original jfr (attached in jbs) which had 3 chunks out of which the last chunk "WLF_3.jfr" with Event type (a) "com.oracle.weblogic.servlet.ServletRequestRunEvent" was causing the NPE. This could be a corrupt event or in complete event captured in the jfr.  
This is a assertive fix and tried debugging the metadata parse (for any Ignored exception) and didn't find any noticeable place which can lead to events not indexed while collecting the events. 

After this fix compared the number of expected events of (a) with "jfr" tool in "jmc" (for WLF_3.jfr there were 32 events in (a) ) and didn't find any missing ones. 

Steps to re-produce NPE : load [WLS_3.jfr](https://bugs.openjdk.java.net/secure/attachment/95852/WLF_3.jfr.lz4)

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Commit messages:
 - 7337: JMC fails to parse JFR with events from WebLogic Server

Changes: https://git.openjdk.java.net/jmc/pull/293/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=293&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7337
  Stats: 7 lines in 1 file changed: 2 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jmc/pull/293.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/293/head:pull/293

PR: https://git.openjdk.java.net/jmc/pull/293