compiler/checker: the compiler panics instead of returning a type-checking error with invalid default statement

[zapateo]

Consider this test:

"https://github.com/open2b/scriggo/issues/941": {
	sources: fstest.Files{
		"index.html": `{% var filters = filters default nil %}`,
	},
	main: native.Package{
		Name: "main",
		Declarations: native.Declarations{
			"filters": (*[]int)(nil),
		},
	},
},

instead of returning an error relative to the use of the untyped nil, it panics the type checker.

Note that this code:

"https://github.com/open2b/scriggo/issues/941": {
	sources: fstest.Files{
		"index.html": `{% var filters = filters default nil %}`,
	},
	expectedBuildErr: "use of untyped nil",
},

works as expected. The problem seems to be related to this code:

scriggo/internal/compiler/checker_assignment.go

Lines 401 to 413 in aeab4b8

	for i, rh := range rhs {
	for j, ti := range rh {
	if ti == nil {
	continue
	}
	if ti.Nil() {
	panic(tc.errorf(subExpr(nodeRhs[i], j == 0), "use of untyped nil"))
	}
	if j == 0 || ti.Untyped() {
	tc.mustBeAssignableTo(ti, subExpr(nodeRhs[i], j == 0), rh[1].Type, false, nil)
	}
	}
	}

[zapateo]

@gazerro take a look at this.