Fix hts_idx_get_n_no_coor() out-of-bounds memory access

For CRAI-indexed CRAM files idx actually points to an hts_cram_idx_t,
which has no n_no_coor field. Return an appropriate "not available"
value instead.

hts.c

@@ -2789,6 +2789,7 @@ int hts_idx_get_stat(const hts_idx_t* idx, int tid, uint64_t* mapped, uint64_t*
 
 uint64_t hts_idx_get_n_no_coor(const hts_idx_t* idx)
 {
+    if (idx->fmt == HTS_FMT_CRAI) return 0;
     return idx->n_no_coor;
 }