RFR: JDK-8256258: some missing NULL checks or asserts after CodeCache::find_blob_unsafe

[Matthias Baesken]

Most return values of CodeCache::find_blob_unsafe are NULL checked or at least handled by an asserts. However especially on ppc a few are missing and should be added (e.g. in nativeInst_ppc.cpp).

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Commit messages:
 - JDK-8256258

Changes: https://git.openjdk.java.net/jdk/pull/1181/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1181&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256258
  Stats: 10 lines in 3 files changed: 7 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1181.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1181/head:pull/1181

PR: https://git.openjdk.java.net/jdk/pull/1181