fix Error while searching for multiple entries with smarty 4.3.4 (#108)

[davidcoutadeur]

fixes #108

[coudot]

In which case would listing_sortby not be an integer?

[davidcoutadeur]

in the case it is not found in the listing of search attributes, then the array_search returns a boolean (false)

I imagined that was the reason for the previous default:0+1

[coudot]

Indeed. Thanks for the fix.