RFR: 8335370: Fix -Wzero-as-null-pointer-constant warning in jvmti_common.hpp

[Kim Barrett]

Please review this trivial change to the print_method function in
test/lib/jdk/test/lib/jvmti/jvmti_common.hpp.  It was passing 0 as the FILE*
argument to fflush, triggering -Wzero-as-null-pointer-constant if that warning
is enabled.  The change is to replace that 0 with nullptr.

This removes about 10% of the -Wzero-as-null-pointer-constant warnings when
building with that option.

Testing: mach5 tier1

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Commit messages:
 - fix fflush call in jvmti_common.hpp

Changes: https://git.openjdk.org/jdk/pull/19962/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19962&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335370
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19962.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19962/head:pull/19962

PR: https://git.openjdk.org/jdk/pull/19962