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

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.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>