[BUG] IKEv2: SIGSEGV in ip4_sas when resolving responder hostname on a system with no IPv4 addresses

[HoneyBBQ]

Environment:

• VPP Version: v25.10-release
• OS: Ubuntu 24.04 (Docker Container)
• Plugins: ikev2, dns, nat

Description:
VPP crashes with SIGSEGV when calling ikev2_initiate_sa_init for a profile configured with a hostname as responder, if the VPP instance has no IPv4 addresses configured on any interface.

Steps to reproduce:

1. Start VPP with dns and ikev2 plugins enabled.
2. Create a loopback interface but do not assign any IP address:

   vppctl loopback create
   vppctl set int state loop0 up

3. Configure an IKEv2 profile with a hostname responder:

   vppctl ikev2 profile add test
   vppctl ikev2 profile set test responder loop0 google.com

4. Initiate the SA:

   vppctl ikev2 initiate sa-init test

Actual Results:
VPP crashes immediately with the following stack trace:

#0  0x00007b623610624f ip4_sas + 0x2f from /lib/x86_64-linux-gnu/libvnet.so.25.10
#1  0x00007b61f48f6571 from /usr/lib/x86_64-linux-gnu/vpp_plugins/dns_plugin.so
#4  0x00007b61f48fc36a dns_resolve_name + 0x4a from /usr/lib/x86_64-linux-gnu/vpp_plugins/dns_plugin.so
#5  0x00007b61f46ea45c from /usr/lib/x86_64-linux-gnu/vpp_plugins/ikev2_plugin.so

Expected Results:
VPP should return an error (e.g., VNET_API_ERROR_NO_SOURCE_ADDRESS) instead of crashing.