Fix segfault when plugin insantiation fails

*PluginInfo::load() eventually calls `plugin->set_info()`
which depends on a valid PluginPtr. The method needs to return
early if `plugin` is unset or null.

libs/ardour/audio_unit.cc

@@ -2624,6 +2624,7 @@ AUPluginInfo::load (Session& session)
 
 		if (!comp->IsValid()) {
 			error << ("AudioUnit: not a valid Component") << endmsg;
+			return PluginPtr ();
 		} else {
 			plugin.reset (new AUPlugin (session.engine(), session, comp));
 		}

libs/ardour/lxvst_plugin.cc

@@ -85,12 +85,11 @@ LXVSTPluginInfo::load (Session& session)
 
 			if (handle == NULL) {
 				error << string_compose(_("LXVST: cannot load module from \"%1\""), path) << endmsg;
-			}
-			else {
+				return PluginPtr ((Plugin*) 0);
+			} else {
 				plugin.reset (new LXVSTPlugin (session.engine(), session, handle, PBD::atoi(unique_id)));
 			}
-		}
-		else {
+		} else {
 			error << _("You asked ardour to not use any LXVST plugins") << endmsg;
 			return PluginPtr ((Plugin*) 0);
 		}

libs/ardour/mac_vst_plugin.cc

@@ -90,12 +90,11 @@ MacVSTPluginInfo::load (Session& session)
 
 			if (handle == NULL) {
 				error << string_compose (_("MacVST: cannot load module from \"%1\""), path) << endmsg;
-			}
-			else {
+				return PluginPtr ((Plugin*) 0);
+			} else {
 				plugin.reset (new MacVSTPlugin (session.engine (), session, handle, PBD::atoi (unique_id)));
 			}
-		}
-		else {
+		} else {
 			error << _("You asked ardour to not use any MacVST plugins") << endmsg;
 			return PluginPtr ((Plugin*) 0);
 		}

libs/ardour/windows_vst_plugin.cc

@@ -85,6 +85,7 @@ WindowsVSTPluginInfo::load (Session& session)
 
 			if (!handle) {
 				error << string_compose(_("VST: cannot load module from \"%1\""), path) << endmsg;
+				return PluginPtr ((Plugin*) 0);
 			} else {
 				plugin.reset (new WindowsVSTPlugin (session.engine(), session, handle, PBD::atoi(unique_id)));
 			}