cli: don't crash when there's no argument

[endothermicdev]

This should provide the default help message and exit, but freeing pointers passed to the default config setup lead to a segmentation fault when help tries to provide usage and triggers the callback.

Program received signal SIGSEGV, Segmentation fault.
__strlen_generic () at ../sysdeps/aarch64/multiarch/../strlen.S:98
98	../sysdeps/aarch64/multiarch/../strlen.S: No such file or directory.
(gdb) bt
#0  __strlen_generic () at ../sysdeps/aarch64/multiarch/../strlen.S:98
#1  0x000000555555b9f0 in opt_show_charp (
    buf=0x7ffffff0a0 "\"Usage: /usr/local/bin/lightning-cli <command> [<params>...]\n--conf=<file>     \"...", len=80, p=0x7ffffff150)
    at ccan/ccan/opt/helpers.c:216
#2  0x000000555555d9b4 in add_desc (
    base=0x555557dff8 "Usage: /usr/local/bin/lightning-cli <command> [<params>...]\n--conf=<file>", ' ' <repeats 13 times>, "Specify configuration file\n", ' ' <repeats 27 times>, "(default: \"Usage: /usr/local/bin/lightning-cli <command> [<p"..., len=0x7ffffff130, 
    max=0x7ffffff128, indent=26, width=137, opt=0x555557dbd0) at ccan/ccan/opt/usage.c:154
#3  0x000000555555df5c in opt_usage (argv0=0x7ffffff687 "/usr/local/bin/lightning-cli", extra=0x55555659b0 "<command> [<params>...]")
    at ccan/ccan/opt/usage.c:232
#4  0x0000005555554674 in main (argc=1, argv=0x7ffffff448) at cli/lightning-cli.c:705

Changelog-Fixed: lightning-cli properly returns help without argument