perf traceevent: Ensure read cmdlines are null terminated.

Issue detected by address sanitizer.

Fixes: cd4ceb63 ("perf util: Save pid-cmdline mapping into tracing header")
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210226221431.1985458-1-irogers@google.com


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