Accept NULL data_in pointer when passing zero input frames
Accept NULL data_in pointer when passing zero input frames

Closes: https://github.com/erikd/libsamplerate/pull/33
Signed-off-by: Erik de Castro Lopo <1586a8e151fb6d548e7a343b9bcdfea42e83ffc0@mega-nerd.com>

erikd/libsamplerate,erikd/libsamplerate,erikd/libsamplerate