stream_encoder.c : Improve fix for arithmetic overflow.
stream_encoder.c : Improve fix for arithmetic overflow.

Only use the 32 bit accumulator if the input data is 16 bits or less.

fredericgermain/flac,waitman/flac,fredericgermain/flac,waitman/flac,waitman/flac,waitman/flac,waitman/flac,fredericgermain/flac,fredericgermain/flac