RFR: 8274286: Skip null for make_referent_alive in referenceProcessor

[Albert Mingkun Yang]

Simple change of avoiding a virtual call for null referent. With this change, two methods of processing the discovered list, `process_discovered_list_work` and `preclean_discovered_reflist`, share the same structure.

Test: hotspot_gc

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Commit messages:
 - referent

Changes: https://git.openjdk.java.net/jdk/pull/5682/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5682&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274286
  Stats: 23 lines in 1 file changed: 11 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5682.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5682/head:pull/5682

PR: https://git.openjdk.java.net/jdk/pull/5682