Improve ly:spawn ()

Improve ly:spawn ()

Issue 4910/3: Avoid crash when g_spawn_sync () fails

  When g_spawn_sync () fails,
  `standard_output` and `standard_error` may not be set.
  This commit makes to check whether or not the value is valid.

Issue 4910/2: Replace the warning message output method in ly_run_command ()

  This commit replaces
  the warning message output method in ly_run_command ()
  from fprintf () to warning ().

Issue 4910/1: Fix memory leak in ly:spawn ()

  This commit makes free the memory allocated by g_spawn_sync ().

[trueroad, 2016-07-07 21:50:11]

I've pushed to staging.

Issue 4910/3: Avoid crash when g_spawn_sync () fails
commit e740e74f900465daf4bffb9361cf42bdbf47ffc8

Issue 4910/2: Replace the warning message output method in ly_run_command ()
commit f97af256ef891d0501c9a6d7985b74a23d7ebe6a

Issue 4910/1: Fix memory leak in ly:spawn ()
commit e5c35162e112e97b1e3562cb2386f76160d48339