Demarshaling messages with zero-length arrays fails

[albel727]

When a message signature contains an array, but the message body actually contains no elements, parsing incorrectly fails with DemarshalError::MismatchedParens, due to invalid handling in

dbus-bytestream/src/demarshal.rs

Lines 185 to 195 in f7f7d1c

	let mut sig_copy = "".to_owned();
	while *offset < start_offset+(array_len as usize) {
	// We want to pass the same signature to each call of demarshal
	sig_copy = sig.to_owned();
	vec.push(try!(demarshal(buf, offset, &mut sig_copy)));
	}
	// Now that we're done with our elements we can forget the elements consumed by demarshal
	let mut mysig = sig.clone();
	mysig.truncate(sig.len() - sig_copy.len());
	mysig.insert(0, 'a');
	*sig = sig_copy;

Note that if the while loop doesn't execute even once, sig_copy will remain empty, and the currently parsed sig will be unconditionally emptied, instead of being set to fields remaining to be parsed.

Maybe I'll fix the bug myself later and publish a PR, but meanwhile here's an example failing test case with an array in the middle of a simple struct. Both nonempty and empty array tests are added for contrast.

albel727@460d6c1