ladspa-sink: Set a silence memchunk for the memblockq
A crash was observed that was caused by pa_memblockq_peek() returning
a NULL memblock in sink_input_pop_cb(). The scenario where this was
happening was
1. Delete 2 rtp-recv's connected to a ladspa-sink
2. Delete ladspa-sink
3. Delete alsa-sink
4. Create alsa-sink
5. Create ladspa-sink
6. Create 2 rtp-recv's connected to the ladspa-sink
The crash was probably caused by a rewind that made the read index go
negative while the write index was at least zero, causing there to be
a gap in the memblockq. The problematic rewind might have been caused
by adding the rtp-recv stream to the ladspa-sink. That has not been
proven, but this looks very similar to the bug that was fixed in
module-virtual-sink in commit
6bd34156b130c07b130de10111a12ef6dab18b52.