avcodec/webp: validate the distance prefix code

avcodec/webp: validate the distance prefix code

According to the WebP Lossless Bitstream Specification the highest
allowed value for a prefix code is 39.

If prefix_code is too large, the calculated extra_bits has an invalid
value and triggers an assertion in get_bits.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 5de2dab12b951b2fe121eb18503accfc91cd1565)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>