perf script: Exception handling when the print fmt is empty

perf script: Exception handling when the print fmt is empty

After collecting samples for events 'syscalls:', perf-script with python
script doesn't occasionally work generating a segmentation fault.

The reason is that the print fmt is empty and a value of
event->print_fmt.args is NULL, so dereferencing the null pointer results
in a segmentation fault i.e.:

    # perf record -e syscalls:*
    # perf script -g python
    # perf script -s perf-script.py

    in trace_begin
    syscalls__sys_enter_brk  3 79841.832099154  3777 test.sh  syscall_nr=12, brk=0

    ... (omitted) ...

    Segmentation fault (core dumped)

For example, a format of sys_enter_getuid() hasn't
print fmt as below.

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_getuid/format
    name: sys_enter_getuid
    ID: 188
    format:
            field:unsigned short common_type;         offset:0; size:2; signed:0;
            field:unsigned char common_flags;         offset:2; size:1; signed:0;
            field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
            field:int common_pid;                     offset:4; size:4; signed:1;
            field:int syscall_nr;                     offset:8; size:4; signed:1;

    print fmt: ""

So add exception handling to avoid this problem.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1456413179-12331-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>