Reject messages with trailing junk

These will never be sent by libwayland and likely indicate that a peer
is using a wrong protocol specification.  Furthermore, there are other
implementations that also reject such messages, so using them is not
portable.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>

doc/publican/sources/Protocol.xml

@@ -113,6 +113,8 @@
 	    The second has 2 parts of 16 bits each.  The upper 16 bits are the message
 	    size in bytes, starting at the header (i.e. it has a minimum value of 8).
 	    The lower is the request/event opcode.  The size must be a multiple of 4.
+	    Messages must be sent using the fewest bytes possible, so padding after
+	    the end of a message is not permitted.
 	  </para>
 	</listitem>
       </itemizedlist>

src/connection.c

@@ -1069,6 +1069,12 @@ wl_connection_demarshal(struct wl_connection *connection,
 		}
 	}
 
+	if (p != end) {
+		wl_log("trailing junk\n");
+		errno = EINVAL;
+		goto err;
+	}
+
 	wl_connection_consume(connection, size);
 
 	return closure;