There are a few places that call round{up|down}_pow_of_two() with the
value zero, and this causes undefined behavior warnings.  Avoid
calling those macros if such a nonsense value is passed; it's a minor
optimization as well, as we handle it as either an error or a value to
be skipped, instead.

Reported-by: syzbot+33ef0b6639a8d2d42b4c@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201218161730.26596-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>