<Sound Dev> RFR: 8266248: Compilation failure in PLATFORM_API_MacOSX_MidiUtils.c with Xcode 12.5

[Sergey Bylokhov]

The new XCode does not like the cast of NULL to the UInt32. The fix replaces NULL with zero.

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Commit messages:
 - Update PLATFORM_API_MacOSX_MidiUtils.c

Changes: https://git.openjdk.java.net/jdk/pull/3808/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3808&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266248
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3808.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3808/head:pull/3808

PR: https://git.openjdk.java.net/jdk/pull/3808