minor optimizations to FLAC__bitreader_read_rice_signed_block()
minor optimizations to FLAC__bitreader_read_rice_signed_block()

fredericgermain/flac,waitman/flac,Distrotech/flac,kode54/flac,Distrotech/flac,kode54/flac,waitman/flac,fredericgermain/flac,kode54/flac,LordJZ/libflac,waitman/flac,LordJZ/libflac,waitman/flac,fredericgermain/flac,LordJZ/libflac,Distrotech/flac,kode54/flac,fredericgermain/flac,waitman/flac,LordJZ/libflac,Distrotech/flac