ALSA: usb-audio: Fix missing error handling for get_min_max*()

ALSA: usb-audio: Fix missing error handling for get_min_max*()

The recent fix to add the error return value check from get_min_max*()
missed one case in build_audio_procunit() where no error value is set.
This may lead to an uninitialized variable and confuse the caller
(although this wouldn't happen practically because err is set for the
loop of num_ins at the beginning of the funciton).

Fix it by setting "err = 0" properly at the missing case, too.

Fixes: 4f55a85cd4fc ("ALSA: usb-audio: Add error checks against get_min_max*()")
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/ad36dGpCBTGsyFr_@stanley.mountain
Link: https://patch.msgid.link/20260414093336.305464-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>