vss: Avoid double free on exit.

vss: Avoid double free on exit.

When para_server fails to receive the fd for the next audio file,
the memory pointed to by mmd->afd.afhi.chunk_table is freed but the
pointer is not set to NULL. If the failure was due to server and afs
receiving SIGINT, the subsequent signal handling code might attempt
to free the chunk table again. This double free error is detected by
glibc, which results resulting in messages such as

	free(): invalid size
	Aborted

Fortunately, this is easy to fix.