avcodec/alsdec: fix max bits in ltp prefix code

The maximum number of bits int the prefix code for
p(0) is 4. By setting it as 3, we were missing the
last 0 bit.
This fixes bug #4715 present on the trac.

Signed-off-by: Umair Khan <omerjerk@gmail.com>
Reviewed-by: Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>