explain mdl_midl_sort() istack size

libraries/libmdb/midl.c

@@ -15,6 +15,7 @@
  * <http://www.OpenLDAP.org/license.html>.
  */
 
+#include <limits.h>
 #include <string.h>
 #include <sys/types.h>
 #include <assert.h>
@@ -134,7 +135,8 @@ int mdb_midl_append( IDL ids, ID id )
 void
 mdb_midl_sort( ID *ids )
 {
-	int istack[16*sizeof(int)];
+	/* Max possible depth of int-indexed tree * 2 items/level */
+	int istack[sizeof(int)*CHAR_BIT * 2];
 	int i,j,k,l,ir,jstack;
 	ID a, itmp;