Null pointer dereference if create_plugin() returns null

[SamWindell]

CLAP plugins are allowed to return null from create_plugin if there's an error: https://github.com/free-audio/clap/blob/69a69252fdd6ac1d06e246d9a04c0a89d9607a17/include/clap/factory/plugin-factory.h#L30-L37

The wrapper should make an effort to handle this.

[baconpaul]

My gut is you start here

void Plugin::connectClap(const clap_plugin_t* clap)
{
  _plugin = clap;

  // initialize the plugin
  if (!_plugin->init(_plugin))
  {
    _plugin->destroy(_plugin);
    _plugin = nullptr;
    return;
  }

in clap_proxy

[defiantnerd]

if the pointer const clap_plugin_t* is a nullptr this means that the clap declared a plugin in its factory but didn't deliver a plugin instance. nevertheless it should not crash.

See PR #401

[defiantnerd]

please check this:
#410

This would really point to a serious bug in the plugin itself - but okay.