wpdb->result instance should be checked `mysqli_num_fields` in `load_col_info()`

[giuseppe.mazzapica]

In wpdb::load_col_info() the function mysqli_num_fields() is called passing $this->result without any check on this property value.

The problem is that function has a type hint to mysqli_result (see ​http://php.net/manual/en/mysqli-result.field-count.php), but $this->result may be false.

Just like in other places in the same class, the function should check that $this->result instanceof mysqli_result before calling mysqli_num_fields().

[pento]

Thanks for the bug report, @giuseppe.mazzapica!

[ocean90]

Introduced in [27250].

Related: #27982

[procodewp]

Checks that this->result is valid before calling mysqli_num_fields() and mysql_num_fields()