fix(xfunc ARRAY filter): protect against digits in IFS

Author: akinomyoga

completions/ARRAY

@@ -48,7 +48,7 @@ _comp_xfunc_ARRAY_filter()
         esac
     done
 
-    shift $((OPTIND - 1))
+    shift "$((OPTIND - 1))"
     if (($# != 2)); then
         printf 'bash_completion: %s: %s\n' "$FUNCNAME" "unexpected number of arguments: $#" >&2
         printf 'usage: %s %s\n' "$FUNCNAME" "[-EFGpsmxrC] ARRAY_NAME CONDITION" >&2