perf archive: Fix filtering of empty build-ids

A non-existent build-id used to be treated as all-zero SHA-1 hash.
Build-ids are now variable width. A non-existent build-id is an empty
string and "perf buildid-list" pads this with spaces. This is true even
when using old perf.data files recorded from older versions of perf;
"perf buildid-list" never reports an all-zero hash anymore.

This fixes "perf-archive" to skip missing build-ids by skipping lines
that start with a padding space rather than with zeroes.

Signed-off-by: Nicholas Fraser <nfraser@codeweavers.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Huw Davies <huw@codeweavers.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ulrich Czekalla <uczekalla@codeweavers.com>
Link: https://lore.kernel.org/r/442bffc7-ac5c-0975-b876-a549efce2413@codeweavers.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>