Ensure GetStringUTFRegion is null-terminated

Null terminate GetStringUTFRegion results. This should help prevent
some types of bugs in JNI.

This changes behavior to match what is observed on the RI.

Test: ./test.py
Bug: 166834353
Change-Id: Ifee6ea2d10ffb7501fc7f141f6da7dc08214fd23
4 files changed