hw/audio/asc: fix SIGSEGV in asc_realize()

hw/audio/asc: fix SIGSEGV in asc_realize()

AUD_open_out() may fail and return NULL. This may then lead to
a segmentation fault in memset() below. The memset() behaviour
is undefined if the pointer to the destination object is a null
pointer.

Add the missing error handling code.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20250515054429.7385-4-vr_qemu@t-online.de>
(cherry picked from commit d009f26a54f573468be721590a19350c224bc730)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>