python: Fix python plugin crash

python: Fix python plugin crash

If python pugin reads and writes collectd values, the write python
plugin callback may crash (90%) on free table var (line 478) if the
meta data structure is created but no information is present there.
It's possible to reproduce it due to changes introduced by #2135 PR.

cpy_write_callback():
 ...
 425   if (value_list->meta) {
 426     char **table;
 427     meta_data_t *meta = value_list->meta;
 428
 429     int num = meta_data_toc(meta, &table);
 430     for (int i = 0; i < num; ++i) {
 ...
 477     }
 478     free(table);
 479   }

Change-Id: Ic897933f447b9280f266b6e78b6b73bd12182eba
Signed-off-by: Mytnyk, VolodymyrX <volodymyrx.mytnyk@intel.com>