RFR: 8062870: src/share/vm/services/mallocTracker.hpp:64 assert(_count > 0) failed: Negative ,counter

[Coleen Phillimore]

Summary: Signed bitfield size y can only have (1 << y)-1 values.

We were overflowing the the _pos index and reusing the 0th element in 
the MallocSiteTable for two different stack traces which caused the 
assert for deallocation.

Tested with nsk.quick.testlist and jtreg runtime tests with 
-XX:NativeMemoryTracking=detail.

open webrev at http://cr.openjdk.java.net/~coleenp/8062870/
bug link https://bugs.openjdk.java.net/browse/JDK-8062870

Thanks,
Coleen