SWQGeneralEvaluator method crashes for a SWQ_ISNULL evaluation

[math]

I've took a look at the code, and the problem is that for this case, we don't have a comparison value, since we just want to look if it's null.

So this code is failing because sub_node_values[1] is NULL or invalid:

sub_node_values[1]->float_value = sub_node_values[1]->int_value;

I've fixed my version this way in swq_op_general.cpp, ln 113:

if( node->nSubExprCount > 1 && sub_node_values[1]->field_type == SWQ_INTEGER )
    sub_node_values[1]->float_value = sub_node_values[1]->int_value;

[rouault]

Duplicate of #4091