RFR(xxs): 8167650: NMT should check for invalid MEMFLAGS.

[Thomas Stüfe]

Hi all,

may I have plase a review for this tiny change? It just adds some assert to
NMT.

Bug: https://bugs.openjdk.java.net/browse/JDK-8167650
webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
MEMFLAGS/webrev.00/webrev/

We had an ugly memory overwrite caused by this - ultimately our fault,
because we fed an invalid memory flag to NMT - but it was difficult to
find. An assert would have saved some time.

Thank you!

Thomas