ffmemless: Dont destroy empty table

[neochapay]

Related #3

if plguin cat load plugin in ffm_setup_effects we destroy table and destroy it second time in ffm_sink_shutdown()

[pvuorela]

To me this looks suspicious, though already currently too. Now if the hash table is destroyed in two places, the latter one would be using a dangling pointer if the instance got deleted already? Assigning null after deleted would protect that, but not sure here why the error case even does the destroy.

cc @jusa

[monich]

void
g_hash_table_destroy (GHashTable *hash_table)
{
  g_hash_table_remove_all (hash_table);
  g_hash_table_unref (hash_table);
}