Fix CVE-2020-12762.

This commit is a squashed backport of the following commits
on the master branch:

  * 099016b7e8
  * 77d935b7ae
  * d07b910149
  * 519dfe1591
  * a59d5acfab

arraylist.c

@@ -135,6 +135,9 @@ array_list_del_idx( struct array_list *arr, size_t idx, size_t count )
 {
 	size_t i, stop;
 
+	/* Avoid overflow in calculation with large indices. */
+	if (idx > SIZE_T_MAX - count)
+		return -1;
 	stop = idx + count;
 	if ( idx >= arr->length || stop > arr->length ) return -1;
 	for ( i = idx; i < stop; ++i ) {