Fix panic in completion of non-existing namespace variable.

This fixes #1209.

pkg/eval/purely_eval.go

@@ -74,7 +74,11 @@ func (ev *Evaler) PurelyEvalPrimary(pn *parse.Primary) interface{} {
 		fm := &Frame{Evaler: ev, local: ev.Global(), up: new(Ns)}
 		ref := resolveVarRef(fm, qname, nil)
 		if ref != nil {
-			return deref(fm, ref).Get()
+			variable := deref(fm, ref)
+			if variable == nil {
+				return nil
+			}
+			return variable.Get()
 		}
 	}
 	return nil

pkg/eval/purely_eval_test.go

@@ -30,6 +30,10 @@ func TestPurelyEvalCompound(t *testing.T) {
 		{code: "foo$x", upto: 3, wantValue: "foo"},
 		{code: "~", wantValue: home},
 		{code: "~/foo", wantValue: home + "/foo"},
+		{code: "$ns:x", wantValue: "foo"},
+
+		{code: "$bad", wantErr: ErrImpure},
+		{code: "$ns:bad", wantErr: ErrImpure},
 
 		{code: "[abc]", wantErr: ErrImpure},
 		{code: "$y", wantErr: ErrImpure},
@@ -38,10 +42,13 @@ func TestPurelyEvalCompound(t *testing.T) {
 	}
 
 	ev := NewEvaler()
-	ev.AddGlobal(NsBuilder{
+	g := NsBuilder{
 		"x": vars.NewReadOnly("bar"),
 		"y": vars.NewReadOnly(vals.MakeList()),
-	}.Ns())
+	}.
+		AddNs("ns", NsBuilder{"x": vars.NewReadOnly("foo")}.Ns()).
+		Ns()
+	ev.AddGlobal(g)
 
 	for _, test := range tests {
 		t.Run(test.code, func(t *testing.T) {