Us the same link instance in send_packet()

[ArisMorgens]

When flashing a Crazyflie 2.0 via cfclient, most of the times the process got stuck in "identifying decks to update", with this error:

crazyflie-lib-python/cflib/crazyflie/__init__.py", line 390, in send_packet
    self.link.send_packet(pk)
    ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'send_packet'

This was caused by send_packet() reading self.link twice: first in if self.link is not None: , and then in elf.link.send_packet(pk). Between these reads, other threads (e.g. close_link()) can set the link to None.

This PR fixes that by reading self.link only once, so the commands in send_packet() will always agree.

[gemenerik]

If another thread sets the link to None, isn't this supposed to fail?
I would say the problem is that we lose the connection while trying to identify the decks.