svq1enc: fix out of bounds reads

level can be 5, but there are only four codebooks.

Fixes ubsan runtime error: index 5 out of bounds for type 'int8_t
[4][96]'

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
