Revert a bug from c1f4b7c where CRAM aux tags were not compressed.

In an attempt to fix memory leaks during encode of malformed CRAM
data, that lead to bailing out early, we attempted to get everything
to a uniform state as soon as possible.  Specifically s->aux_blocks
get migrated to s->blocks earlier, so if we bail out we don't have the
decision to make as to which bit of memory needs freeing.

That worked, but unfortunately I forgot that the compression is
applied to s->aux_blocks and not s->blocks for auxiliary tags, so
moving the blocks earlier gives uncompressed blocks in CRAM.  The
format is valid and all tests pass, but it's overly large for obvious
reasons.

Sadly this is a rather serious bug caused by an attempt to fix a
really minor bug.

Fixes samtools/samtools#1968

cram/cram_encode.c

@@ -953,14 +953,14 @@ static int cram_compress_slice(cram_fd *fd, cram_container *c, cram_slice *s) {
      */
     {
         int i;
-        for (i = 0; i < s->naux_block; i++) {
-            if (!s->aux_block[i] || s->aux_block[i] == s->block[0])
+        for (i = DS_END /*num_blk - naux_blk*/; i < s->hdr->num_blocks; i++) {
+            if (!s->block[i] || s->block[i] == s->block[0])
                 continue;
 
-            if (s->aux_block[i]->method != RAW)
+            if (s->block[i]->method != RAW)
                 continue;
 
-            if (cram_compress_block2(fd, s, s->aux_block[i], s->aux_block[i]->m,
+            if (cram_compress_block2(fd, s, s->block[i], s->block[i]->m,
                                      method, level))
                 return -1;
         }