Follow the recommended order of operations

As suggested in
    samtools/htslib#1823 (comment)
first comes hts_idx_destroy() followed by sam_close()

mpileup2/mpileup.c

@@ -399,9 +399,9 @@ void mpileup_destroy(mpileup_t *mplp)
     int i;
     for (i=0; i<mplp->nbam; i++)
     {
+        if ( mplp->bam[i].iter ) hts_itr_destroy(mplp->bam[i].iter);
         sam_close(mplp->bam[i].fp);
         if ( mplp->bam[i].cached_rec ) bam_destroy1(mplp->bam[i].cached_rec);
-        if ( mplp->bam[i].iter ) hts_itr_destroy(mplp->bam[i].iter);
     }
     for (i=0; i<mplp->nbam_names; i++) free(mplp->bam_names[i]);
     free(mplp->bam);