ssl: Don't ignore non-ragged EOF

Testing NEO/go client wrt NEO/py server revealed a bug in NEO/py SSL
handling: proper non-ragged EOF from a peer is ignored, and so leads to
hang in infinite loop inside _SSL.receive with read_buf memory growing
indefinitely. Details are below:

NEO/py wraps raw sockets with

	ssl.wrap_socket(suppress_ragged_eofs=False)

which instructs SSL layer to convert unexpected EOF when receiving a TLS
record into SSLEOFError exception. However when remote peer properly
closes its side of the connection, socket.read() still returns b'' to
report non-ragged regular EOF:

https://github.com/python/cpython/blob/v2.7.18/Lib/ssl.py#L630-L650

The code was handling SSLEOFError but not b'' return from socket recv.
Thus after NEO/go client was disconnecting and properly closing its side
of the connection, the code started to loop indefinitely in _SSL.receive
under `while 1` with  b'' returned by self.socket.recv() appended to
read_buf again and again.

-> Fix it by detecting non-ragged EOF as well and, similarly to how
SSLEOFError is handled, converting them into self._error('recv', None).

neo/lib/connector.py

@@ -278,7 +278,12 @@ class _SSL:
     def receive(self, read_buf):
         try:
             while 1:
-                read_buf.append(self.socket.recv(4096))
+                data = self.socket.recv(4096)
+                if data == b'':
+                    # non-ragged EOF (peer properly closed its side of connection)
+                    self._error('recv', None)
+                    return
+                read_buf.append(data)
         except ssl.SSLWantReadError:
             pass
         except socket.error, e: