Fix to avoid technically undefined behavior.
The C standard says that calling library functions (including memcpy) with invalid arguments (including a NULL pointer) is undefined behavior unless otherwise noted (which memcpy doesn't). op_filter_read_native() invokes op_read_native() with NULL for the _pcm buffer, which triggers such a memcpy invocation. Even though it should be perfectly fine in practice to pass NULL to memcpy when copying zero bytes, don't do it. Thanks to a person who did not wish to be credited for the report.
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