perf lock contention: Add a missing NULL check

perf lock contention: Add a missing NULL check

I got a report for a failure in BPF verifier on a recent kernel with
perf lock contention command.  It checks task->sighand->siglock without
checking if sighand is NULL or not.  Let's add one.

  ; if (&curr->sighand->siglock == (void *)lock)
  265: (79) r1 = *(u64 *)(r0 +2624)     ; frame1: R0_w=trusted_ptr_task_struct(off=0,imm=0)
                                        ;         R1_w=rcu_ptr_or_null_sighand_struct(off=0,imm=0)
  266: (b7) r2 = 0                      ; frame1: R2_w=0
  267: (0f) r1 += r2
  R1 pointer arithmetic on rcu_ptr_or_null_ prohibited, null-check it first
  processed 164 insns (limit 1000000) max_states_per_insn 1 total_states 15 peak_states 15 mark_read 5
  -- END PROG LOAD LOG --
  libbpf: prog 'contention_end': failed to load: -13
  libbpf: failed to load object 'lock_contention_bpf'
  libbpf: failed to load BPF skeleton 'lock_contention_bpf': -13
  Failed to load lock-contention BPF skeleton
  lock contention BPF setup failed
  lock contention did not detect any lock contention

Fixes: 1811e82767dcc ("perf lock contention: Track and show siglock with address")
Reviewed-by: Ian Rogers <irogers@google.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240409225542.1870999-1-namhyung@kernel.org