aaccoder: prevent crash of anmr coder

If minq is negative, the range of sf_idx can be larger than
SCALE_MAX_DIFF allows, causing assertion failures later in
encode_scale_factors.

Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>