bgpd: Notice when we unlock if we should NULL pointer

bgpd: Notice when we unlock if we should NULL pointer

The bi->net pointer that is being unlocked had a commit
that removed the `bi->net = NULL;` recently.  This code
was preventing a use after free crash being experienced
in other code paths.  While commit 37e679629f9 was fixing
a different code path crash.

Make the parent->net pointer aware it may be locked/freed
from multiple places and to not NULL the pointer to it
unless we have actually freed the data.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>