ALSA: usb-audio: Check mixer unit bitmap yet more strictly

The bmControls (for UAC1) or bmMixerControls (for UAC2/3) bitmap has a
variable size depending on both input and output pins.  Its size is to
fit with input * output bits.  The problem is that the input size
can't be determined simply from the unit descriptor itself but it
needs to parse the whole connected sources.  Although the
uac_mixer_unit_get_channels() tries to check some possible overflow of
this bitmap, it's incomplete due to the lack of the  evaluation of
input pins.

For covering possible overflows, this patch adds the bitmap overflow
check in the loop of input pins in parse_audio_mixer_unit().

Fixes: 0bfe5e43

 ("ALSA: usb-audio: Check mixer unit descriptors more strictly")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>