perf intel-bts: Fix potential NULL pointer dereference found by the smatch tool

Based on the following report from Smatch, fix the potential NULL
pointer dereference check.

  tools/perf/util/intel-bts.c:898
  intel_bts_process_auxtrace_info() error: we previously assumed
  'session->itrace_synth_opts' could be null (see line 894)

  tools/perf/util/intel-bts.c:899
  intel_bts_process_auxtrace_info() warn: variable dereferenced before
  check 'session->itrace_synth_opts' (see line 898)

  tools/perf/util/intel-bts.c
  894         if (session->itrace_synth_opts && session->itrace_synth_opts->set) {
  895                 bts->synth_opts = *session->itrace_synth_opts;
  896         } else {
  897                 itrace_synth_opts__set_default(&bts->synth_opts,
  898                                 session->itrace_synth_opts->default_no_sample);
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
  899                 if (session->itrace_synth_opts)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  900                         bts->synth_opts.thread_stack =
  901                                 session->itrace_synth_opts->thread_stack;
  902         }

'session->itrace_synth_opts' is impossible to be a NULL pointer in
intel_bts_process_auxtrace_info(), thus this patch removes the NULL test
for 'session->itrace_synth_opts'.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20190708143937.7722-3-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>