MDL-80286 datafield_date: cast field content to real when sorting.

This ensures we don't under/overflow the size of an integer in DB.

mod/data/field/date/field.class.php

@@ -198,7 +198,7 @@ class data_field_date extends data_field_base {
 
     function get_sort_sql($fieldname) {
         global $DB;
-        return $DB->sql_cast_char2int($fieldname, true);
+        return $DB->sql_cast_char2real($fieldname, true);
     }
 
     /**