Make ti-stress work on RI

In some cases the RI will send JVMTI agents events with some of the
required reference arguments null. This could cause the ti-stress
agent to crash. Fix this by explicitly null checking before calling
DeleteLocalRef.

Test: java -agentpath:$ANDROID_HOST_OUT/nativetest64/libtistress.so=jvmti-stress,trace \
           HelloWorld

Change-Id: I29f4144e2e773253b76f5fa8b38d57ae1deab03b