Crash with particular command sequence in grammar_sandbox

[jeybb]

Describe the bug
Two commands in particular sequence crash grammer_sandbox.

(put "x" in "[ ]" if you already tried following)
[x] Did you check if this is a duplicate issue?
[x] Did you test it on the latest FRRouting/frr master branch?

To Reproduce
Steps to reproduce the behavior:

gdb -q .libs/grammar_sandbox
(gdb) r

test# c t
test(config)# s

               Program received signal SIGSEGV, Segmentation fault.
                                                                   0x00007ffff7af7aa4 in nb_cli_show_config_cmds (vty=0x657640, config=0x0, with_defaults=<optimized out>) at lib/northbound_cli.c:451
451             vty_out(vty, "frr defaults %s\n", DFLT_NAME);

(gdb) bt
#0  0x00007ffff7af7aa4 in nb_cli_show_config_cmds (vty=0x657640, config=0x0, with_defaults=<optimized out>) at lib/northbound_cli.c:451
#1  nb_cli_show_config (vty=0x657640, config=0x0, format=NB_CFG_FMT_CMDS, translator=<optimized out>, with_defaults=false) at lib/northbound_cli.c:502
#2  0x00007ffff7af78d2 in show_config_candidate_section_magic (argc=0, self=<optimized out>, vty=<optimized out>, argv=<optimized out>) at lib/northbound_cli.c:848
#3  show_config_candidate_section (self=<optimized out>, vty=0x657640, argc=<optimized out>, argv=<optimized out>) at ./lib/northbound_cli_clippy.c:455
#4  0x00007ffff7a7f7b2 in cmd_execute_command_real (vline=<optimized out>, filter=FILTER_RELAXED, vty=<optimized out>, cmd=<optimized out>) at lib/command.c:1059
#5  0x00007ffff7a7f003 in cmd_execute_command (vline=0x656cb0, vty=0x657640, cmd=0x0, vtysh=0) at lib/command.c:1119
#6  0x00007ffff7a7fb2c in cmd_execute (vty=0x657640, cmd=<optimized out>, matched=0x0, vtysh=0) at lib/command.c:1272
#7  0x00007ffff7b70b3f in vty_command (vty=<optimized out>, buf=0x658cd0 "s") at lib/vty.c:528
#8  vty_execute (vty=0x657640) at lib/vty.c:1297
#9  0x00007ffff7b6ad8e in vty_read (thread=<optimized out>) at lib/vty.c:1510
#10 0x00007ffff7b59405 in thread_call (thread=0x7fffffffe260) at lib/thread.c:1603
#11 0x0000000000401524 in main (argc=<optimized out>, argv=<optimized out>) at lib/grammar_sandbox_main.c:66

Expected behavior
Error message

Versions

• OS Kernel: Ubuntu 18.04
• FRR Version: git checkout