[mythtv-cmyth] Fix memory leak in MythProgramInfo::StorageGroup

The function cmyth_proginfo_storagegroup increases the reference counter
using ref_hold. Therefore we have to decrease it again with ref_release.

This issue was introduced in d06317d39a9c65239eb8cf909bec821c44bfd0eb.

3,520 bytes in 110 blocks are definitely lost in loss record 6 of 8
  at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  by 0x55F5FE: __ref_alloc (alloc.c:275)
  by 0x55F835: ref_strdup (alloc.c:412)
  by 0x55BB48: __cmyth_rcv_proginfo (socket.c:2235)
  by 0x55CB89: __cmyth_rcv_proglist (socket.c:2642)
  by 0x5536C8: cmyth_proglist_get_list (proglist.c:271)
  by 0x553D2C: cmyth_proglist_get_all_recorded (proglist.c:329)
  by 0x50510A: MythConnection::GetRecordedPrograms() (MythConnection.cpp:237)
  by 0x4ED1CB: PVRClientMythTV::FillRecordings() (pvrclient-mythtv.cpp:721)
  by 0x4EEF83: PVRClientMythTV::GetRecordings(ADDON_HANDLE_STRUCT*) (pvrclient-mythtv.cpp:521)