perf session: Fix possible null pointer dereference in session.c

cppcheck detected following warning:
[tools/perf/util/session.c:1628] -> [tools/perf/util/session.c:1632]:
 (warning) Possible null pointer dereference: session - otherwise it
 is redundant to check it against null.

In order to avoide null pointer, check the pointer before use.
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Link: http://lkml.kernel.org/r/1400087618-13628-1-git-send-email-standby24x7@gmail.comSigned-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/util/session.c

@@ -1625,13 +1625,14 @@ int perf_session__cpu_bitmap(struct perf_session *session,
 void perf_session__fprintf_info(struct perf_session *session, FILE *fp,
 				bool full)
 {
-	int fd = perf_data_file__fd(session->file);
 	struct stat st;
-	int ret;
+	int fd, ret;
 
 	if (session == NULL || fp == NULL)
 		return;
 
+	fd = perf_data_file__fd(session->file);
+
 	ret = fstat(fd, &st);
 	if (ret == -1)
 		return;