perf annotate: Fix a bug of division by zero when calculating percent

perf annotate: Fix a bug of division by zero when calculating percent

Currently perf-annotate with --print-line can print
-nan(0x8000000000000) because of division by zero when calculating
percent. The division by zero happens when a sum of samples is zero in
symbol__get_source_line(), so fix it.

For example:

After running 'perf record' like below,

    $ perf record -e "{cycles,page-faults,branch-misses}" ./a.out

Before:

    $ perf annotate --stdio -l

  Sorted summary for file /home/taeung/workspace/a.out
  ----------------------------------------------

   32.89    -nan    7.04 a.c:38
   25.14    -nan    0.00 a.c:34
   16.26    -nan   56.34 a.c:31
   15.88    -nan    1.41 a.c:37
    5.67    -nan    0.00 a.c:39
    1.13    -nan   35.21 a.c:26
    0.95    -nan    0.00 a.c:44
    0.57    -nan    0.00 a.c:32
   Percent                 |      Source code & Disassembly of a.out for cycles (529 samples)
  -----------------------------------------------------------------------------------------
                         :
  ...

   a.c:26    0.57    -nan    4.23 :         40081a:       mov    %edi,-0x24(%rbp)
   a.c:26    0.00    -nan    9.86 :         40081d:       mov    %rsi,-0x30(%rbp)

  ...

However, if a sum of samples is zero (e.g. 'page-faults'),
skip calculating percent.

After:

    $ perf annotate --stdio -l

  Sorted summary for file /home/taeung/workspace/a.out
  ----------------------------------------------

   32.89    0.00    7.04 a.c:38
   25.14    0.00    0.00 a.c:34
   16.26    0.00   56.34 a.c:31
   15.88    0.00    1.41 a.c:37
    5.67    0.00    0.00 a.c:39
    1.13    0.00   35.21 a.c:26
    0.95    0.00    0.00 a.c:44
    0.57    0.00    0.00 a.c:32
   Percent                 |      Source code & Disassembly of old for cycles (529 samples)
  -----------------------------------------------------------------------------------------
                         :
  ...

  a.c:26    0.57    0.00    4.23 :         40081a:       mov    %edi,-0x24(%rbp)
  a.c:26    0.00    0.00    9.86 :         40081d:       mov    %rsi,-0x30(%rbp)

  ...

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1490598638-13947-3-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>