RFR: 8335298: Fix -Wzero-as-null-pointer-constant warning in G1CardSetContainers

[Kim Barrett]

Please review this change to the G1CardSetInlinePtr default constructor to
avoid -Wzero-as-null-pointer-constant.  The problem is that it is casting a
constant 0 to a pointer.  Because casting 0 to a pointer is what this code
needs to do, the options are to (1) suppress the warning, or (2) write the
code in such a way as to dodge the warning.  We take the latter approach, as
it involves less source code clutter.  We expect an optimizing compiler to
produce the same code either way.

Also simplified the constructors by using delegation.

Testing: mach5 tier1.
Locally (linux-x64) verified the warning no longer occurs with this change.

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Commit messages:
 - fix G1CardSetInlinePtr

Changes: https://git.openjdk.org/jdk/pull/19955/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19955&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335298
  Stats: 15 lines in 1 file changed: 10 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19955.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19955/head:pull/19955

PR: https://git.openjdk.org/jdk/pull/19955