perf record: Fix segfault with --off-cpu when debuginfo is not enabled

perf record: Fix segfault with --off-cpu when debuginfo is not enabled

When kernel is built without debuginfo, running 'perf record' with
--off-cpu results in segfault as below:

   ./perf record --off-cpu -e dummy sleep 1
   libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
   libbpf: failed to find '.BTF' ELF section in /lib/modules/6.13.0-rc3+/build/vmlinux
   libbpf: failed to find valid kernel BTF
   Segmentation fault (core dumped)

The backtrace pointed to:

   #0  0x00000000100fb17c in btf.type_cnt ()
   #1  0x00000000100fc1a8 in btf_find_by_name_kind ()
   #2  0x00000000100fc38c in btf.find_by_name_kind ()
   #3  0x00000000102ee3ac in off_cpu_prepare ()
   #4  0x000000001002f78c in cmd_record ()
   #5  0x00000000100aee78 in run_builtin ()
   #6  0x00000000100af3e4 in handle_internal_command ()
   #7  0x000000001001004c in main ()

Code sequence is:

   static void check_sched_switch_args(void)
   {
        struct btf *btf = btf__load_vmlinux_btf();
        const struct btf_type *t1, *t2, *t3;
        u32 type_id;

        type_id = btf__find_by_name_kind(btf, "btf_trace_sched_switch",
                                         BTF_KIND_TYPEDEF);

btf__load_vmlinux_btf() fails when CONFIG_DEBUG_INFO_BTF is not enabled.

Here bpf__find_by_name_kind() calls btf__type_cnt() with NULL btf value
and results in segfault.

To fix this, add a check to see if btf is not NULL before invoking
bpf__find_by_name_kind().

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://lore.kernel.org/r/20241223135813.8175-1-atrajeev@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>