Problem: zsock_recv 'm' assumes initialized message

This is inconsistent with all other picture formats and leads to
surprising code.

Solution: always initialize message before reading off socket.

If we need an "append to message" picture that should be a special
case, e.g. using 'M' as picture.
Merge pull request #749 from hintjens/master

Problem: zsock_recv 'm' assumes initialized message