bgpd: Fix crash when going down

bgpd: Fix crash when going down

When bgp is sent a SIGINT signal with vrf's configured
bgp proceeds to crash on the way down.  This is because
the bm->bgp list was deleted( but the pointer was kept around)
and then later in the going down process vrf_terminate is called
which attempts to find any associated processes.  Since memory
has been freed and subsuquently used for something else
we get a corrupt bgp pointer and bgp crashes.

Here is the trace:

(gdb) bt
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
    at sigevent.c:255
    at thread.c:1315
(gdb)

Multiple protocols besides bgp and zebra running are needed to make
this happen as well.  So a weird timing related issue?

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