CRM-14317 fix for : while looking into this issue, found a DB: NAN field not found

I didn't replicated the actual issue but, had found this issue.

The issue : <code>$minLatitude</code>, <code>$maxLatitude</code>, <code>$minLongitude</code>, <code>$maxLongitude</code> the value stored in these variable is sometimes 'NAN' due to which, DB: Field not found mysql error is raised which result listing after search.

The fix: added <code>is_nan</code> during the clause building of <code>geo_code</code> where clause.