The on_preview callback gets called with NULL if the buffer in the queue is
too small, so we need to handle the case where the array is NULL. Also
there is a bug in the android source which makes it drop one of the buffers
so if we had 5 buffers, and we renegotiate to a higher resolution, then we'd
only get 4 calls to on_preview_frame with NULL, with one being dropped.
This means we can't reallocate the buffers in the if (data == NULL) case
because we might end up with 0 buffers in the end.
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