Crash due to multiple expressions on filters.assign

[GGuidiRontani]

With pdal-parallelizer 2.0.3 on Windows, both with single and dir options, multiples requests with filters.assign make pdal-parallelizer crash. For exemple,

    {
        "type":"filters.assign",
        "value":
		[
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]",
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]",
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]"
		]
    },

has to be replaced by,

    {
        "type":"filters.assign",
        "value":
		[
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]"
		]
    },    {
        "type":"filters.assign",
        "value":
		[
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]"
		]
    },    {
        "type":"filters.assign",
        "value":
		[
		"value" : "Dimension = ValueExpression [WHERE ConditionalExpression)]"
		]
    },