[jdk8u-dev] RFR: 8317291: Missing null check for nmethod::is_native_method()

[KIRIYAMA Takuya]

I added null check to nmethod::is_native_method() and is_java_method().
I verified that the JVM crash problem no longer occurs by this fix.
I don't attach the test for this fix beause I could reproduce this problem only with debug build.

Would anyone review this fix, pelase?

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Commit messages:
 - 8317291: Missing null check for nmethod::is_native_method()

Changes: https://git.openjdk.org/jdk8u-dev/pull/379/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=379&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317291
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/379.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/379/head:pull/379

PR: https://git.openjdk.org/jdk8u-dev/pull/379