perf tools: Initialize output buffer in build_id__sprintf

We display garbage for undefined build_id objects, because we don't
initialize the output buffer.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20201101233103.3537427-1-jolsa@kernel.org


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