Fixing PrintWriter not being disposed

When there is an exception being thrown, the getStackTrace method of
JNIHelp will initialize a PrintWriter to print the exception message.
But the instance of the PrintWriter will not be disposed, because
PrintWriter will hold the string reference, and finally the local
reference table of VM will overflow. Then system will crash.

The change disposes the PrintWriter from the local reference table
to prevent the overflow.

Change-Id: Ie8dcb3d941034e6e1f480196373189f2d152cf79