segfault when adding OSPF virtual link on Bird 1.6.4

[Cybertinus]

Hello everybody,

Today I was experimenting with a Bird setup. All the Bird machines are 
CentOS 7.6 VMs running in Virtualbox on my laptop. When I add an OSPF 
virtual link to hook up two area 0's together I get a segfault of Bird 
1.6.4 itself. All the links between the VMs are defined as "Internal 
Network" links. Each connection between routers got it's own internal 
network, so there should be no overlap between those connections.

Steps to reproduce:
1. Create a bird.conf with the following contents specified in the 
attached bird.conf
2. Start Bird
3. Add the following line to the area 1 config:
        virtual link 2.2.2.2;
4. Run `birdc configure`
5. Observe the following error in your syslog:
Dec  8 21:01:35 r3 kernel: bird[3469]: segfault at 32 ip 
0000000000429fe0 sp 00007fff599d3350 error 4 in bird[400000+73

I've ran Bird 1.6.4 in a gdb session with the debug symbols installed 
and I created a stack trace after the crash. This gives the output which 
can be found in the attached gdb.txt.
What is interesting to note is that the segfault only occurs when I 
reconfigure Bird. I can start it just fine right after the crash and the 
OSPF sessions come back online and everything.

I hope this gives some useful information about what is happening. If 
you need any more information please don't hesitate to ask. It is a test 
network, so I can tell you absolutely everything about it, nothing is a 
company secret or something like that :).

Kind regards,
Cybertinus
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