perf annotate: Initialize 'arch' variable not to trip some -Werror=maybe-uninitialized

perf annotate: Initialize 'arch' variable not to trip some -Werror=maybe-uninitialized

In some older distros the build is failing due to
-Werror=maybe-uninitialized, in this case we know that this isn't the
case because 'arch' gets initialized by evsel__get_arch(), so make sure
it is initialized to NULL before returning from evsel__get_arch(), as
suggested by Ian Rogers.

E.g.:

    32    17.12 opensuse:15.5                 : FAIL gcc version 7.5.0 (SUSE Linux)
        util/annotate.c: In function 'hist_entry__get_data_type':
    util/annotate.c:2269:15: error: 'arch' may be used uninitialized in this function [-Werror=maybe-uninitialized]
      struct arch *arch;
                   ^~~~
    cc1: all warnings being treated as errors

      43     7.30 ubuntu:18.04-x-powerpc64el    : FAIL gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)
    util/annotate.c: In function 'hist_entry__get_data_type':
    util/annotate.c:2351:36: error: 'arch' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       if (map__dso(ms->map)->kernel && arch__is(arch, "x86") &&
                                        ^~~~~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors

Suggested-by: Ian Rogers <irogers@google.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/CAP-5=fUqtjxAsmdGrnkjhUTLHs-JvV10TtxyocpYDJK_+LYTiQ@mail.gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>