conf: Fix invalid free at parse_args()

conf: Fix invalid free at parse_args()

The previous fix for memory leaks introduced a few regression.
The major one is the assert hit in the error path reaching with NULL
or uninitialized sub object.  Also, in other code paths, it's possible
that an already released sub object gets freed again.

Fix those bugs by initializing the sub object properly and add a NULL
check before calling snd_config_delete().

Fixes: ad5f255b4767 ("conf: fix memory leak on the error path in parse_args()")
Reported-and-tested-by: Mark Hills <mark@xwax.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>