libperf threadmap: Don't segv for index 0 for the NULL 'struct perf_thread_map' pointer

perf_thread_map__nr() returns length 1 if the perf_thread_map is NULL,
meaning index 0 is valid.

When perf_thread_map__pid() of index 0 is read then return the expected
"any" -1 value.

Assert this is only done for index 0.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Gautam Menghani <gautam@linux.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250519195148.1708988-2-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>