RFR (XS) 8025550 valgrind: Conditional jump depends on uninitialised value in Arena::set_size_in_bytes()

[Zhengyu Gu]

This is a simple fix to NMT, which can mis-count arena size due to uninitialized  value. In rare case, if uninitialized size happens to be the same value as arena's initial size, NMT will mis-count the size, since NMT only updates size when the new size does not equal to the old one. 


Bug: https://bugs.openjdk.java.net/browse/JDK-8025550
Webrev:  http://cr.openjdk.java.net/~zgu/8025550/webrev.00/

Thanks,

-Zhengyu