perf auxtrace: Avoid redundant NULL check in auxtrace_mmap_params__set_idx()

perf auxtrace: Avoid redundant NULL check in auxtrace_mmap_params__set_idx()

Since commit eead8a011477 ("libperf threadmap: Don't segv for index 0 for the
NULL 'struct perf_thread_map' pointer"), perf_thread_map__pid() can
check for a NULL map and return -1 if idx is 0. Cleanup
auxtrace_mmap_params__set_idx() and remove the redundant NULL check.

No functional change intended.

Reviewed-by: Aditya Bodkhe <adityab1@linux.ibm.com>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>