Fix AttributeError on LocalBuffer.__delete__ call in python xla client

[neerajprad]

While playing around with some custom JAX primitives, I was noticing the python xla client throwing exceptions like below towards the end (probably when LocalBuffer is gc'd or in a separate cleanup operation). The exceptions are ignored, but they are logged many times in the code that I am working with. This adds a small check (taken from the Executable class) to ensure that c_api hasn't already been freed in the LocalBuffer.delete() method.

Exception ignored in: <bound method LocalBuffer.__del__ of <jaxlib.xla_client.LocalBuffer object at 0x1267a55c0>>
Traceback (most recent call last):
  File "/Users/npradhan/miniconda3/envs/numpyro/lib/python3.6/site-packages/jaxlib/xla_client.py", line 393, in __del__
  File "/Users/npradhan/miniconda3/envs/numpyro/lib/python3.6/site-packages/jaxlib/xla_client.py", line 377, in delete
  File "/Users/npradhan/miniconda3/envs/numpyro/lib/python3.6/site-packages/jaxlib/xla_client.py", line 104, in delete_buffer
AttributeError: 'NoneType' object has no attribute 'DeleteLocalShapedBuffer'

cc. @hawkinsp

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[neerajprad]

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