reflect: call of reflect.Value.Elem on zero Value

[zapateo]

"https://github.com/open2b/scriggo/issues/858": {
	sources: fstest.Files{
		"index.html": `
			{% import pkg "imported.html" %}
			{% ref := &pkg.V %}
			{% *ref = 42 %}
			{{ pkg.PrintV() }}
		`,
		"imported.html": `
			{% var V int %}
			{% macro PrintV %}V is {{ V }}{% end macro %}
		`,
	},
},

NOTE: before proceeding, wait for the merge of the PR #856.