libsubcmd: Silence compiler warning

Initialize the pointer 'o' in options__order to NULL to prevent a
compiler warning/error which is observed when compiling with the '-Og'
option, but is not emitted by the compiler with the current default
compilation options.

For example, when compiling libsubcmd with

$ make "EXTRA_CFLAGS=-Og" -C tools/lib/subcmd/ clean all

Clang version 17.0.6 and GCC 13.3.1 fail to compile parse-options.c due
to following error:

parse-options.c: In function ‘options__order’:
parse-options.c:832:9: error: ‘o’ may be used uninitialized [-Werror=maybe-uninitialized]
832 | memcpy(&ordered[nr_opts], o, sizeof(*o));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
parse-options.c:810:30: note: ‘o’ was declared here
810 | const struct option *o, *p = opts;
| ^
cc1: all warnings being treated as errors

Signed-off-by: Eder Zulian <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/subcmd/parse-options.c

···
 static struct option *options__order(const struct option *opts)
 {
 	int nr_opts = 0, nr_group = 0, nr_parent = 0, len;
-	const struct option *o, *p = opts;
+	const struct option *o = NULL, *p = opts;
 	struct option *opt, *ordered = NULL, *group;
 
 	/* flatten the options that have parents */