perf buildid-list: We need to check if a file is ELF first

I.e. before we try to use it as a perf.data file by calling
perf_session__new, otherwise we lose the feature that shows the
build id for the given ELF file, this one:

[root@sandy redhat-perfdata-mtech-15]# perf buildid-list -i /root/.debug/.build-id/97/54896de655b6ac088ec2bf5113b35c06f72709
9754896de655b6ac088ec2bf5113b35c06f72709
[root@sandy redhat-perfdata-mtech-15]# perf buildid-list -i /lib/libc-2.12.so
38adaeff4f7c21899b13b28c1a2e6c199ca4c744
[root@sandy redhat-perfdata-mtech-15]#

Regression introduced in:

efad1415 "perf report: Accept fifos as input file"

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-3ktgyg83fwpqyfpoj0t2ezp0@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-buildid-list.c

@@ -49,18 +49,16 @@ static int perf_session__list_build_ids(bool force, bool with_hits)
 	struct perf_session *session;
 
 	symbol__elf_init();
-
-	session = perf_session__new(input_name, O_RDONLY, force, false,
-				    &build_id__mark_dso_hit_ops);
-	if (session == NULL)
-		return -1;
-
 	/*
 	 * See if this is an ELF file first:
 	 */
-	if (filename__fprintf_build_id(session->filename, stdout))
+	if (filename__fprintf_build_id(input_name, stdout))
 		goto out;
 
+	session = perf_session__new(input_name, O_RDONLY, force, false,
+				    &build_id__mark_dso_hit_ops);
+	if (session == NULL)
+		return -1;
 	/*
 	 * in pipe-mode, the only way to get the buildids is to parse
 	 * the record stream. Buildids are stored as RECORD_HEADER_BUILD_ID
@@ -69,8 +67,8 @@ static int perf_session__list_build_ids(bool force, bool with_hits)
 		perf_session__process_events(session, &build_id__mark_dso_hit_ops);
 
 	perf_session__fprintf_dsos_buildid(session, stdout, with_hits);
-out:
 	perf_session__delete(session);
+out:
 	return 0;
 }