RFR: 8345050: Fix -Wzero-as-null-pointer warning in MemPointer ctor

[Kim Barrett]

Please review this trivial change to use nullptr instead of a literal 0 in a
call to Node::dump_bfs by the MemPointer ctor.

Testing: mach5 tier1

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Commit messages:
 - fix backsliding

Changes: https://git.openjdk.org/jdk/pull/22388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345050
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22388/head:pull/22388

PR: https://git.openjdk.org/jdk/pull/22388