perf tools: Don't try to lookup objdump for live mode

Arnaldo reported that annotation during perf top resulted in a segfault.
It was because the env->arch was NULL and we don't set it for a live
session.  In fact, no need to look up objdump in this case since we can
use system's default (native) objdump.
Reported-by: NArnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: NNamhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Irina Tirdea <irina.tirdea@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1352251815-12615-1-git-send-email-namhyung@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/common.c

@@ -199,6 +199,13 @@ static int perf_session_env__lookup_binutils_path(struct perf_session_env *env,
 
 int perf_session_env__lookup_objdump(struct perf_session_env *env)
 {
+	/*
+	 * For live mode, env->arch will be NULL and we can use
+	 * the native objdump tool.
+	 */
+	if (env->arch == NULL)
+		return 0;
+
 	return perf_session_env__lookup_binutils_path(env, "objdump",
 						      &objdump_path);
 }