RFR(S): 8192978: Missing checks and small fixes in jdwp library

[Langer, Christoph]

Hi,

I'd like to contribute a few fixes that have been done in our JDK builds that 
add a few additional checks and cleanups. Can I please get a review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8192978
WebRev: http://cr.openjdk.java.net/~clanger/webrevs/8192978.0/

Thanks,
Christoph