opus_pvq: improve PVQ search for low Ks

Since the probelm mentioned only happened when the phase was negative
(e.g. the sum had to be decreased), only discarding dimensions with a
zero pulse in that case restored the search's previously low distortion
at low Ks when the phase is never negative.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>
