Correctly decode empty arrays and maps

Fixes #42.

Author: guelfey

dbus.go

@@ -177,6 +177,8 @@ func alignment(t reflect.Type) int {
 		return 4
 	case signatureType:
 		return 1
+	case interfacesType: // sometimes used for structs
+		return 8
 	}
 	switch t.Kind() {
 	case reflect.Uint8:

decoder.go

@@ -171,6 +171,8 @@ func (dec *decoder) decode(s string, depth int) interface{} {
 				panic(FormatError("input exceeds container depth limit"))
 			}
 			length := dec.decode("u", depth).(uint32)
+			// Even for empty maps, the correct padding must be included
+			dec.align(8)
 			spos := dec.pos
 			for dec.pos < spos+int(length) {
 				dec.align(8)
@@ -188,6 +190,8 @@ func (dec *decoder) decode(s string, depth int) interface{} {
 		}
 		length := dec.decode("u", depth).(uint32)
 		v := reflect.MakeSlice(reflect.SliceOf(typeFor(s[1:])), 0, int(length))
+		// Even for empty arrays, the correct padding must be included
+		dec.align(alignment(typeFor(s[1:])))
 		spos := dec.pos
 		for dec.pos < spos+int(length) {
 			ev := dec.decode(s[1:], depth+1)