bgpd: fix LU label callback crash

bgpd: fix LU label callback crash

under some conditions, the callback to get a label for
a LU bgp path could be called after the path had already
been freed. In this case we would be reading garbage
and potentially crash. Lock the path info before
queueing the callback, and unlock as the first step
of the callback, exiting gracefully if the path info
is now NULL.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>