bgpd: fix null pointer dereference bug

bgpd: fix null pointer dereference bug

If path->net is NULL in the bgp_path_info_free() function, then
bgpd would crash in bgp_addpath_free_info_data() with the following
backtrace:

 (gdb) bt
 #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
 #1  0x00007ff7b267a42a in __GI_abort () at abort.c:89
 #2  0x00007ff7b39c1ca0 in core_handler (signo=11, siginfo=0x7ffff66414f0, context=<optimized out>) at lib/sigevent.c:249
 #3  <signal handler called>
 #4  idalloc_free_to_pool (pool_ptr=pool_ptr@entry=0x0, id=3) at lib/id_alloc.c:368
 #5  0x0000560096246688 in bgp_addpath_free_info_data (d=d@entry=0x560098665468, nd=0x0) at bgpd/bgp_addpath.c:100
 #6  0x00005600961bb522 in bgp_path_info_free (path=0x560098665400) at bgpd/bgp_route.c:252
 #7  bgp_path_info_unlock (path=0x560098665400) at bgpd/bgp_route.c:276
 #8  0x00005600961bb719 in bgp_path_info_reap (rn=rn@entry=0x5600986b2110, pi=pi@entry=0x560098665400) at bgpd/bgp_route.c:320
 #9  0x00005600961bf4db in bgp_process_main_one (safi=SAFI_MPLS_VPN, afi=AFI_IP, rn=0x5600986b2110, bgp=0x560098587320) at bgpd/bgp_route.c:2476
 #10 bgp_process_wq (wq=<optimized out>, data=0x56009869b8f0) at bgpd/bgp_route.c:2503
 #11 0x00007ff7b39d5fcc in work_queue_run (thread=0x7ffff6641e10) at lib/workqueue.c:294
 #12 0x00007ff7b39ce3b1 in thread_call (thread=thread@entry=0x7ffff6641e10) at lib/thread.c:1606
 #13 0x00007ff7b39a3538 in frr_run (master=0x5600980795b0) at lib/libfrr.c:1011
 #14 0x000056009618a5a3 in main (argc=3, argv=0x7ffff6642078) at bgpd/bgp_main.c:481

Add a null-check protection to fix this problem.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>