stream_encoder.c : Fix an arithmetic overflow in the RICE2 partitioner.
For a specific 24 bit WAV file provided by Leigh Dyer http://lists.xiph.org/pipermail/flac-dev/2013-July/004284.html encoding with compression level 7 was generating a file a couple of orders of magintude larger than the original. Debugging showed that variable abs_residual_partition_sum (a FLAC__uint32) in function precompute_partition_info_sums_() was suffering from an arithmetic overflowing on some 24 bit input files although this value overflowing did not always cause larger output files. Since the value abs_residual_partition_sum is eventually stored in an array of FLAC__uint64, it makes sense to make abs_residual_partition_sum a FLAC__uint64 anyway. Debugging this problem was made easier by use of the Clang compiler's -fsanitize=integer option.