RFR: JDK-8274435: EXCEPTION_ACCESS_VIOLATION in BFSClosure::closure_impl

[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.

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Commit messages:
 - JDK-8274435: EXCEPTION_ACCESS_VIOLATION in BFSClosure::closure_impl

Changes: https://git.openjdk.java.net/jdk/pull/5736/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5736&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274435
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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