Fix panic in interpolate_walk

Verify that we have enough containers in the stack to look for a map in
replaceCurrent.

config/config_test.go

@@ -204,8 +204,8 @@ func TestConfigValidate_table(t *testing.T) {
 		{
 			"nested types in variable default",
 			"validate-var-nested",
-			true,
-			"ERROR",
+			false,
+			"",
 		},
 	}
 

config/interpolate_walk.go

@@ -206,6 +206,12 @@ func (w *interpolationWalker) Primitive(v reflect.Value) error {
 }
 
 func (w *interpolationWalker) replaceCurrent(v reflect.Value) {
+	// if we don't have at least 2 values, we're not going to find a map, but
+	// we could panic.
+	if len(w.cs) < 2 {
+		return
+	}
+
 	c := w.cs[len(w.cs)-2]
 	switch c.Kind() {
 	case reflect.Map: