RFR: 8335688: Fix -Wzero-as-null-pointer-constant warnings from fflush calls in jvmti tests

[Kim Barrett]

Please review this change to some jvmti tests, which were calling fflush with
an argument of 0. Most of these are in C++ code, where we change them to use
nullptr as the argument.  A couple are in C, where we change them to use NULL.
This removes a bunch of -Wzero-as-null-pointer-constant when building with
that enabled.

Testing: mach5 tier1

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Commit messages:
 - fix fflush calls in hotspot code

Changes: https://git.openjdk.org/jdk/pull/20032/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20032&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335688
  Stats: 175 lines in 25 files changed: 0 ins; 0 del; 175 mod
  Patch: https://git.openjdk.org/jdk/pull/20032.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20032/head:pull/20032

PR: https://git.openjdk.org/jdk/pull/20032