Fix double plugin unload crash on meta retry

[GAMMACASE]

Prevent double plugin unload on meta retry if the plugin has failed to load on that retry attempt.

Current retry logic checks for plugin load state after a call to _Load, and unloads the plugin if it failed to load

metamod-source/core/metamod_plugins.cpp

Lines 303 to 329 in ccce734

	CPlugin *pl = _Load((*i)->m_File.c_str(), Pl_Console, error, len);
	if (!pl)
	{
	return false;
	}
	if (pl->m_Status >= Pl_Paused)
	{
	//Now it gets crazy... unload the original copy.
	_Unload( (*i), true, buffer, sizeof(buffer)-1 );
	//Set the new copy's id
	pl->m_Id = id;
	//We just wasted an id... reclaim it
	m_LastId--;
	return true;
	}
	else
	{
	//don't really care about the buffer here
	_Unload(pl, true, buffer, sizeof(buffer)-1);
	//We just wasted an id... reclaim it
	m_LastId--;
	return false;
	}

But _Load method by itself checks for plugin load state and performs unload if it was unsuccessful

metamod-source/core/metamod_plugins.cpp

Lines 599 to 604 in ccce734

	if (pl->m_Lib && (pl->m_Status < Pl_Paused))
	{
	pl->m_Events.clear();
	UnregAllConCmds(pl);
	g_SourceHook.UnloadPlugin(pl->m_Id, new Unloader(pl, false));
	}

This causes a double unload if a plugin has failed to load on that retry attempt, thus causing ReadyToUnload to be called twice, where on a second go, plugin_->m_Lib would be nullptr and results in a crash on linux

metamod-source/core/metamod_plugins.cpp

Lines 398 to 416 in ccce734

	void ReadyToUnload(SourceHook::Plugin plug)
	{
	if (plugin_->m_UnloadFn != NULL)
	plugin_->m_UnloadFn();
	dlclose(plugin_->m_Lib);
	if (destroy_)
	{
	delete plugin_;
	}
	else
	{
	plugin_->m_Lib = NULL;
	plugin_->m_API = NULL;
	}
	delete this;
	}

This pr both, removes the double unload on retry and adds a safeguard on dlclose in ReadyToUnload method to check for potential nullptr.