RFR: 8287363: null pointer should use NULL instead of 0

[Yasumasa Suenaga]

We found using `0` as `NULL` in java_md_common.c . `0` is not a pointer, so we should use `NULL` where we want to handle it.

https://github.com/openjdk/jdk/pull/8646#discussion_r882294076

Also I found using `0` as NUL char in java_md_common.c , so I replaced it to `\0` in this PR.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into JDK-8287363
 - 8287363: null pointer should use NULL instead of 0

Changes: https://git.openjdk.java.net/jdk/pull/8931/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8931&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287363
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8931.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8931/head:pull/8931

PR: https://git.openjdk.java.net/jdk/pull/8931