Fix a race condition in socket constructor.

There was a race where the socket connect callbacks could get called
before the socket was added to the global cache.  Then, in the callback,
the socket would try to be retrieved from the cache, but it wouldn't be
there, resulting in an error.

This was a pretty bad race, and it affected real code. This was brought
up on #40 by GitHub user
chaoflow.  Many thanks to him for identifying the fix!

pynng/nng.py

@@ -300,13 +300,16 @@ def __init__(self, *,
             check_err(lib.nng_pipe_notify(
                 self.socket, event, lib._nng_pipe_cb, as_void))
 
+        # The socket *must* be added to the _live_sockets map before calling
+        # listen/dial so that no callbacks are called before the socket is
+        # added to the map (because then the callback would fail!).
+        _live_sockets[id(self)] = self
+
         if listen is not None:
             self.listen(listen)
         if dial is not None:
             self.dial(dial, block=block_on_dial)
 
-        _live_sockets[id(self)] = self
-
     def dial(self, address, *, block=None):
         """Dial the specified address.