pimd: fix crash when configuring ssmpingd

Command: `ip ssmpingd 1.1.1.1`

Backtrace:
```
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
0x00007fd1d3b02859 in __GI_abort () at abort.c:79
0x00007fd1d3e323e1 in yang_dnode_xpath_get_canon (dnode=<optimized out>, xpath_fmt=<optimized out>, ap=<optimized out>) at lib/yang_wrappers.c:61
0x00007fd1d3e34f41 in yang_dnode_get_ipv4 (addr=addr@entry=0x7ffc368554d4, dnode=<optimized out>, xpath_fmt=xpath_fmt@entry=0x5556af8680d4 "./source-addr") at lib/yang_wrappers.c:826
0x00005556af8216d3 in routing_control_plane_protocols_control_plane_protocol_pim_address_family_ssm_pingd_source_ip_create (args=0x7ffc36855530) at pimd/pim_nb_config.c:925
0x00007fd1d3dec13f in nb_callback_create (nb_node=0x5556b197ea40, nb_node=0x5556b197ea40, errmsg_len=8192, errmsg=0x7ffc36855a90 "", resource=0x5556b18fa6f8, dnode=0x5556b1ad7a10, event=NB_EV_APPLY, context=0x5556b1ad75c0) at lib/northbound.c:1260
nb_callback_configuration (context=0x5556b1ad75c0, event=NB_EV_APPLY, change=<optimized out>, errmsg=0x7ffc36855a90 "", errmsg_len=8192) at lib/northbound.c:1648
0x00007fd1d3deca6c in nb_transaction_process (event=event@entry=NB_EV_APPLY, transaction=transaction@entry=0x5556b1ad75c0, errmsg=errmsg@entry=0x7ffc36855a90 "", errmsg_len=errmsg_len@entry=8192) at lib/northbound.c:1779
0x00007fd1d3decdd6 in nb_candidate_commit_apply (transaction=0x5556b1ad75c0, save_transaction=save_transaction@entry=true, transaction_id=transaction_id@entry=0x0, errmsg=errmsg@entry=0x7ffc36855a90 "", errmsg_len=errmsg_len@entry=8192) at lib/northbound.c:1129
0x00007fd1d3decf15 in nb_candidate_commit (context=..., candidate=<optimized out>, save_transaction=save_transaction@entry=true, comment=comment@entry=0x0, transaction_id=transaction_id@entry=0x0, errmsg=0x7ffc36855a90 "", errmsg_len=8192) at lib/northbound.c:1162
0x00007fd1d3ded4af in nb_cli_classic_commit (vty=vty@entry=0x5556b1ada2a0) at lib/northbound_cli.c:50
0x00007fd1d3df025f in nb_cli_apply_changes_internal (vty=vty@entry=0x5556b1ada2a0, xpath_base=xpath_base@entry=0x7ffc36859b50 ".", clear_pending=clear_pending@entry=false) at lib/northbound_cli.c:177
0x00007fd1d3df06ad in nb_cli_apply_changes (vty=vty@entry=0x5556b1ada2a0, xpath_base_fmt=xpath_base_fmt@entry=0x0) at lib/northbound_cli.c:233
0x00005556af80fdd5 in pim_process_ssmpingd_cmd (vty=0x5556b1ada2a0, operation=NB_OP_CREATE, src_str=0x5556b1ad9630 "1.1.1.1") at pimd/pim_cmd_common.c:3423
0x00007fd1d3da7b0e in cmd_execute_command_real (vline=vline@entry=0x5556b1ac9520, vty=vty@entry=0x5556b1ada2a0, cmd=cmd@entry=0x0, up_level=up_level@entry=0) at lib/command.c:982
0x00007fd1d3da7cb1 in cmd_execute_command (vline=vline@entry=0x5556b1ac9520, vty=vty@entry=0x5556b1ada2a0, cmd=0x0, vtysh=vtysh@entry=0) at lib/command.c:1040
0x00007fd1d3da7e50 in cmd_execute (vty=vty@entry=0x5556b1ada2a0, cmd=cmd@entry=0x5556b1ae0a30 "ip ssmpingd 1.1.1.1", matched=matched@entry=0x0, vtysh=vtysh@entry=0) at lib/command.c:1207
0x00007fd1d3e278be in vty_command (vty=vty@entry=0x5556b1ada2a0, buf=<optimized out>) at lib/vty.c:591
0x00007fd1d3e27afd in vty_execute (vty=0x5556b1ada2a0) at lib/vty.c:1354
0x00007fd1d3e2bb23 in vtysh_read (thread=<optimized out>) at lib/vty.c:2362
0x00007fd1d3e22254 in event_call (thread=thread@entry=0x7ffc3685cd80) at lib/event.c:2003
0x00007fd1d3dce9e8 in frr_run (master=0x5556b183c830) at lib/libfrr.c:1218
0x00005556af803653 in main (argc=6, argv=<optimized out>, envp=<optimized out>) at pimd/pim_main.c:162
```

Signed-off-by: Igor Ryzhov <[email protected]>
(cherry picked from commit 8ffc364)

pimd/pim_nb_config.c

@@ -962,8 +962,7 @@ int routing_control_plane_protocols_control_plane_protocol_pim_address_family_ss
 	case NB_EV_APPLY:
 		vrf = nb_running_get_entry(args->dnode, NULL, true);
 		pim = vrf->info;
-		yang_dnode_get_pimaddr(&source_addr, args->dnode,
-				       "./source-addr");
+		yang_dnode_get_pimaddr(&source_addr, args->dnode, NULL);
 		result = pim_ssmpingd_start(pim, source_addr);
 		if (result) {
 			snprintf(
@@ -993,8 +992,7 @@ int routing_control_plane_protocols_control_plane_protocol_pim_address_family_ss
 	case NB_EV_APPLY:
 		vrf = nb_running_get_entry(args->dnode, NULL, true);
 		pim = vrf->info;
-		yang_dnode_get_pimaddr(&source_addr, args->dnode,
-				       "./source-addr");
+		yang_dnode_get_pimaddr(&source_addr, args->dnode, NULL);
 		result = pim_ssmpingd_stop(pim, source_addr);
 		if (result) {
 			snprintf(