perf evsel: Make perf_evsel__name() accept a NULL argument

perf evsel: Make perf_evsel__name() accept a NULL argument

In which case it simply returns "unknown", like when it can't figure out
the evsel->name value.

This makes this code more robust and fixes a problem in 'perf trace'
where a NULL evsel was being passed to a routine that only used the
evsel for printing its name when a invalid syscall id was passed.

Reported-by: Leo Yan <leo.yan@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-f30ztaasku3z935cn3ak3h53@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>