unix-manager: fix cppcheck errors

This patch fixes the following errors:
 [src/unix-manager.c:306]: (error) Memory pointed to by 'client' is freed twice.
 [src/unix-manager.c:313]: (error) Memory pointed to by 'client' is freed twice.
 [src/unix-manager.c:323]: (error) Memory pointed to by 'client' is freed twice.
 [src/unix-manager.c:334]: (error) Memory pointed to by 'client' is freed twice.

Unix manager was treating the packet after closing the socket if message was
too long.

src/unix-manager.c

@@ -297,6 +297,7 @@ int UnixCommandAccept(UnixCommand *this)
         SCLogInfo("Command server: client message is too long, "
                   "disconnect him.");
         close(client);
+        return 0;
     }
     buffer[ret] = 0;