RFR: 8372442: 2 Null pointer dereference defect groups in jvmtiExport.cpp

[Leonid Mesnik]

The `javaVFrame *jvf = thread->last_java_vframe(&reg_map);`
might be null potentially. Shouldn't be null when called from 
post_field_modification_by_jni but assert doesn't hurt.


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Commit messages:
 - removed completely useless assertions
 - 8372442: 2 Null pointer dereference defect groups in jvmtiExport.cpp

Changes: https://git.openjdk.org/jdk/pull/31352/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=31352&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372442
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/31352.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/31352/head:pull/31352

PR: https://git.openjdk.org/jdk/pull/31352