lavfi/abuffer: init the data planes with NULL pointers.

Samples buffer ref is allocated and loaded with the uninitialized data
pointers:
     av_asrc_buffer_add_buffer()
  -> av_asrc_buffer_add_samples()
  -> avfilter_get_audio_buffer_ref_from_arrays(data, ...)

...which leads to a crash with at least lavfi/ashowinfo in case of !NULL
(see the for loop while samplesref->data[plane]).
