vshCommandParse: Don't leak @tkdata
    
    When parsing cmd line which has "--" on it, this is leaked.
    Problem is, parser->getNextArg() allocates new string and stores
    it into tkdata. But as soon as "--" is detected 'continue' is
    issued without any free of the allocated memory.
    
      ==5304== 3 bytes in 1 blocks are definitely lost in loss record 1 of 782
      ==5304==    at 0x4C2AF50: malloc (vg_replace_malloc.c:299)
      ==5304==    by 0x8BB5AA9: strdup (strdup.c:42)
      ==5304==    by 0x55842CA: virStrdup (virstring.c:941)
      ==5304==    by 0x172B21: _vshStrdup (vsh.c:162)
      ==5304==    by 0x175E8E: vshCommandArgvGetArg (vsh.c:1622)
      ==5304==    by 0x17551D: vshCommandParse (vsh.c:1418)
      ==5304==    by 0x175F25: vshCommandArgvParse (vsh.c:1638)
      ==5304==    by 0x130940: virshParseArgv (virsh.c:820)
      ==5304==    by 0x130C49: main (virsh.c:922)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>