avcodec/g729dec: require buf_size to be non 0

The 0 case was added with the support for multiple packets. It
appears unintended and causes extra complexity and out of array
accesses (though within padding)

No testcase

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f64be9da)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>