OMXNodeInstance: Do not use already freed memory.

There is a possibility that other process might
try to access memory that was already freed by freeNode().
To fix this, first mHandle will be checked and only
then access will be allowed.

Test: forcibly kill app many times during omx is alive.
Bug: 113141946

Change-Id: Ib979d3b89cc90ddd7d4805eb0e4f0603a5406c98