Fix cram_encode fuzzer issue caused by negative reference lengths.

Introduced in 24e4e31 with the cache of the LN_length field.  We now
validate this as positive.  The check in cram_encode.c should now not
be necessary, but it's fixed in two places incase we ever get this
cropping up via another route.

Credit to OSS_Fuzz
Fixes oss-fuzz issue 382922241

cram/cram_decode.c

@@ -3247,6 +3247,10 @@ cram_slice *cram_next_slice(cram_fd *fd, cram_container **cp) {
                 if (fd->ooc)
                     break;
 
+//                printf("%p %d:%ld-%ld vs %d:%ld-%ld\n", fd,
+//                       c_next->ref_seq_id, c_next->ref_seq_start, c_next->ref_seq_start+c_next->ref_seq_span-1,
+//                       fd->range.refid, fd->range.start, fd->range.end);
+
                 /* Skip containers not yet spanning our range */
                 if (fd->range.refid != -2 && c_next->ref_seq_id != -2) {
                     // ref_id beyond end of range; bail out

cram/cram_encode.c

@@ -3691,7 +3691,8 @@ static int process_one_read(cram_fd *fd, cram_container *c,
             return -1;
         }
         fake_qual = spos;
-        cr->aend = no_ref ? apos : MIN(apos, c->ref_end);
+        // Protect against negative length refs (fuzz 382922241)
+        cr->aend = no_ref ? apos : MIN(apos, MAX(0, c->ref_end));
         if (cram_stats_add(c->stats[DS_FN], cr->nfeature) < 0)
             goto block_err;
 

cram/cram_io.c

@@ -2790,9 +2790,13 @@ static int refs_from_header(cram_fd *fd) {
             if ((tag = sam_hrecs_find_key(ty, "M5", NULL)))
                 r->ref_id[j]->fn = string_dup(r->pool, tag->str+3);
 
-            if ((tag = sam_hrecs_find_key(ty, "LN", NULL)))
+            if ((tag = sam_hrecs_find_key(ty, "LN", NULL))) {
                 // LN tag used when constructing consensus reference
                 r->ref_id[j]->LN_length = strtoll(tag->str+3, NULL, 0);
+                // See fuzz 382922241
+                if (r->ref_id[j]->LN_length < 0)
+                    r->ref_id[j]->LN_length = 0;
+            }
         }
 
         k = kh_put(refs, r->h_meta, r->ref_id[j]->name, &n);
@@ -5818,6 +5822,8 @@ int cram_set_voption(cram_fd *fd, enum hts_fmt_option opt, va_list args) {
     case CRAM_OPT_RANGE: {
         int r = cram_seek_to_refpos(fd, va_arg(args, cram_range *));
         pthread_mutex_lock(&fd->range_lock);
+//        printf("opt range noseek to %p %d:%ld-%ld\n",
+//               fd, fd->range.refid, fd->range.start, fd->range.end);
         if (fd->range.refid != -2)
             fd->required_fields |= SAM_POS;
         pthread_mutex_unlock(&fd->range_lock);