AAC encoder: fix OOB access in search_for_pns

Fix OOB access in search_for_pns which was using
w2 outside the window group loop, and fix a typo
in which it was checking sf_idx instead of band_type

Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
