lavfi: fix avfilter_default_get_audio_buffer() after changes in av_samples_alloc()

av_samples_alloc() behavior changed in bbb46f3ec, resulting in random
data filling the data[] and linesize[] arrays of the returned
AVFilterBufferRef, which was resulting in wrong behavior in case of code
checking on data[i] nullity.

In particular fixes crash in avfilter_filter_samples():
        for (i = 0; samplesref->data[i]; i++)
            memcpy(link->cur_buf->data[i], samplesref->data[i], samplesref->linesize[0]);

and correctly fills the linesize[] array for planar data.
