node-proxy: try to acquire lock early in reporter's loop

node-proxy: try to acquire lock early in reporter's loop

The lock should be acquired early in this loop.

If the lock gets acquired by another call after we enter that condition *and*
before Reporter.loop() actually acquires it, it can lead to issue if during
this short amount of time the value of `data_ready` gets modified

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>