Replace audio_calloc() with the equivalent g_new0().

With a n_structs argument of 1, g_new0() never returns NULL.
Also remove the unnecessary NULL checks.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230121094735.11644-5-vr_qemu@t-online.de>