8195690: JNI GetObjectRefType doesn't handle NULL
Summary: Properly handle NULL, add some non-NULL preconditions.
Reviewed-by: dholmes, mdoerr
^build/
^dist/
^.idea/
nbproject/private/
^webrev
^.src-rev$
^.jib/
\.DS_Store
\.metadata/
\.recommenders/
test/nashorn/script/external
test/nashorn/lib
NashornProfile.txt
.*/JTreport/.*
.*/JTwork/.*