SSE2 SIMD: Fix prog Huffman enc. error if Sl%16==0

(regression introduced by 5b177b3cab)

The SSE2 implementation of progressive Huffman encoding performed
extraneous iterations when the scan length was a multiple of 16.

Based on:
bb7f1ef983

Fixes #335
Closes #367

testimages/test.scan

@@ -0,0 +1,5 @@
+0 1 2: 0 0 0 0;
+0: 1 16 0 0;
+0: 17 63 0 0;
+1: 1 63 0 0;
+2: 1 63 0 0;