RFR: 8282295: SymbolPropertyEntry::set_method_type fails with assert

[Coleen Phillimore]

This is somewhat trivial change to zero out OopHandle in fullgc_alot_dummy_array.  Other OopHandle releases are for data that is deallocated or not reused so zeroing _obj is not needed in those places.  I didn't want to zero _obj in OopHandle::release anyway without zeroing it in the analogous WeakHandle::release. WeakHandle release code operates on const objects, also which are deallocated after release.
Tested with tier1-4.

-------------

Commit messages:
 - 8282295: SymbolPropertyEntry::set_method_type fails with assert

Changes: https://git.openjdk.java.net/jdk/pull/7730/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7730&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282295
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7730.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7730/head:pull/7730

PR: https://git.openjdk.java.net/jdk/pull/7730