OpenJ9 JVM properly terminates the thread before hdfsThreadDestructor is
invoked. JNIEnv is a mirror of J9VMThread in OpenJ9. After proper thread
termination, accessing JNIEnv in hdfsThreadDestructor (*env)->GetJavaVM,
yields a SIGSEGV since *env is NULL after thread cleanup is performed.
The main purpose of hdfsThreadDestructor is to invoke
DetachCurrentThread, which performs thread cleanup in OpenJ9. Since
OpenJ9 performs thread cleanup before hdfsThreadDestructor is invoked,
hdfsThreadDestructor should account for *env == NULL and skip
DetachCurrentThread.
Signed-off-by: Babneet Singh <sbabneet@ca.ibm.com>