Check the conn in nil before closing if failed connect()

It's possible that the reason connect failed is that the connection
dropped/was reset and the conn is already nil, calling Close() on a
nil conn causes a panic, so we're now checking that the conn is
actually not nil before trying to close it and run the rest of the
failed to connect code.

resolves eclipse-paho#94

Author: alsm

client.go

@@ -190,8 +190,10 @@ func (c *client) Connect() Token {
 
 				rc = c.connect()
 				if rc != packets.Accepted {
-					c.conn.Close()
-					c.conn = nil
+					if c.conn != nil {
+						c.conn.Close()
+						c.conn = nil
+					}
 					//if the protocol version was explicitly set don't do any fallback
 					if c.options.protocolVersionExplicit {
 						ERROR.Println(CLI, "Connecting to", broker, "CONNACK was not CONN_ACCEPTED, but rather", packets.ConnackReturnCodes[rc])