ALSA: usb-audio: Fix possible NULL pointer dereference in snd_usb_pcm_has_fixed_rate()

ALSA: usb-audio: Fix possible NULL pointer dereference in snd_usb_pcm_has_fixed_rate()

[ Upstream commit 5d25e540dca06deff0fbb365823bb9a5254ba08e ]

The subs function argument may be NULL, so do not use it before the NULL check.

Fixes: e320fa10939a ("ALSA: usb-audio: Always initialize fixed_rate in snd_usb_find_implicit_fb_sync_format()")
Reported-by: coverity-bot <keescook@chromium.org>
Link: https://lore.kernel.org/alsa-devel/202301121424.4A79A485@keescook/
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230113085311.623325-1-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>