Add guard against NPE

[cassgenerator]

It is possible that GetDirectBufferAddress returns null in some cases. Given this it would be best to guard against attempting to access the _storedBitmapPixels variable on a null pointer.

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I could only trigger this issue in a kinda race condition in my application, when I would switch to a new view while an Async task was operating on JniBitmaps. I think this caused the underlying memory to be cleaned up by the system and it would then crash when trying to get the bitmap next.

As far as I can tell this guards against that issue, it no longer crashes for me. However it has been a while since I've done any C++. So let me know if anything needs changing.

[AndroidDeveloperLB]

I think you are correct:
https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html#GetDirectBufferAddress

Returns the starting address of the memory region referenced by the buffer. Returns NULL if the memory region is undefined, if the given object is not a direct java.nio.Buffer, or if JNI access to direct buffers is not supported by this virtual machine.

[AndroidDeveloperLB]

Please also check if there are similar cases that I didn't protect against.

[cassgenerator]

I'm chasing another crash, perhaps it is related. Hopefully I track it down.

[AndroidDeveloperLB]

Thank you for contributing.