RFR: 8297801: printnm crashes with invalid address due to null pointer dereference

[Damon Fenacci]

`printnm` crashes if you pass an invalid address. This is caused by a null pointer dereference.
Checking for NULL reference before checking if blob is a method.

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Commit messages:
 - JDK-8297801: printnm crashes with invalid address due to null pointer dereference

Changes: https://git.openjdk.org/jdk/pull/11697/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11697&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297801
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11697.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11697/head:pull/11697

PR: https://git.openjdk.org/jdk/pull/11697