resolve_btfids: Fix compiler warnings

Initialize 'set' and 'set8' pointers to NULL in sets_patch to prevent
possible compiler warnings which are issued for various optimization
levels, but do not happen when compiling with current default
compilation options.

For example, when compiling resolve_btfids with

$ make "HOSTCFLAGS=-O2 -Wall" -C tools/bpf/resolve_btfids/ clean all

Clang version 17.0.6 and GCC 13.3.1 issue following
-Wmaybe-uninitialized warnings for variables 'set8' and 'set':

In function ‘sets_patch’,
inlined from ‘symbols_patch’ at main.c:748:6,
inlined from ‘main’ at main.c:823:6:
main.c:163:9: warning: ‘set8’ may be used uninitialized [-Wmaybe-uninitialized]
163 | eprintf(1, verbose, pr_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.c:729:17: note: in expansion of macro ‘pr_debug’
729 | pr_debug("sorting addr %5lu: cnt %6d [%s]\n",
| ^~~~~~~~
main.c: In function ‘main’:
main.c:682:37: note: ‘set8’ was declared here
682 | struct btf_id_set8 *set8;
| ^~~~
In function ‘sets_patch’,
inlined from ‘symbols_patch’ at main.c:748:6,
inlined from ‘main’ at main.c:823:6:
main.c:163:9: warning: ‘set’ may be used uninitialized [-Wmaybe-uninitialized]
163 | eprintf(1, verbose, pr_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.c:729:17: note: in expansion of macro ‘pr_debug’
729 | pr_debug("sorting addr %5lu: cnt %6d [%s]\n",
| ^~~~~~~~
main.c: In function ‘main’:
main.c:683:36: note: ‘set’ was declared here
683 | struct btf_id_set *set;
| ^~~

Signed-off-by: Eder Zulian <ezulian@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20241022172329.3871958-2-ezulian@redhat.com