RFR: 8302905: arm32 Raspberry Pi OS build broken by JDK-8301494

[David Holmes]

Trivial fix that replaces `nullptr` with 0 in `intptr_t` expressions. This seems more appropriate than casting `nullptr` to `intptr_t`.

Tested with local ARM32 build.

Thanks.

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Commit messages:
 - 8302905: arm32 Raspberry Pi OS build broken by JDK-8301494

Changes: https://git.openjdk.org/jdk/pull/12679/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302905
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12679.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12679/head:pull/12679

PR: https://git.openjdk.org/jdk/pull/12679