RFR: JDK-8274435: EXCEPTION_ACCESS_VIOLATION in BFSClosure::closure_impl [v2]

[Ekaterina Vergizova]

> Hi,
> please review this small change that prevents EXCEPTION_ACCESS_VIOLATION on some Windows machines.
> The crash is intermittently reproducible on win32 client jvm when running e.g. jdk/jfr/jcmd/TestJcmdDumpPathToGCRoots.java test.
> It is caused by BitSet::get_fragment_bits method which returns null for the newly initialized BitSet if granule (= addr >> _bitmap_granularity_shift) is equal to 0.
> 
> Testing:
> jdk/jfr/* with win32 and win64 builds, no crashes or regression after the fix.

Ekaterina Vergizova has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8274435: EXCEPTION_ACCESS_VIOLATION in BFSClosure::closure_impl

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5736/files
  - new: https://git.openjdk.java.net/jdk/pull/5736/files/3fd8ccec..1dfa4c08

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5736&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5736&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5736.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5736/head:pull/5736

PR: https://git.openjdk.java.net/jdk/pull/5736