RFR: 8255563: Missing NULL checks after JDK-8233624

[David Holmes]

JDK-8233624 introduced a check for illegal native method names that will result in the `NativeLookup::*_jni_name` methods returning NULL. JVMCI `registerNativeMethods` uses these APIs without any check for a NULL return, however the methods being registered are part of the JVMCI implementation so should never have an illegal name, so adding a guarantee suffices to deal with that (see comments in JBS issue).

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Commit messages:
 - Fixed JDK-8255563

Changes: https://git.openjdk.java.net/jdk/pull/1068/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1068&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255563
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1068.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1068/head:pull/1068

PR: https://git.openjdk.java.net/jdk/pull/1068