end the campaign properly when a mission is skipped

The "skip training" button and the "exit loop" button should both check whether the campaign is actually over before attempting to load the next mission.  The fix is simple - just use the same logic as in `mission_campaign_skip_to_next`.

Fixes #4412.

Author: Goober5000

code/mission/missioncampaign.cpp

@@ -1822,7 +1822,12 @@ void mission_campaign_exit_loop()
 
 	// set things up for next mission
 	mission_campaign_next_mission();
-	gameseq_post_event(GS_EVENT_START_GAME);
+
+	if ( Campaign.next_mission == -1 || (The_mission.flags[Mission::Mission_Flags::End_to_mainhall]) ) {
+		gameseq_post_event( GS_EVENT_MAIN_MENU );
+	} else {
+		gameseq_post_event( GS_EVENT_START_GAME );
+	}
 }
 
 

code/missionui/missionbrief.cpp

@@ -351,7 +351,7 @@ void brief_skip_training_pressed()
 	mission_campaign_eval_next_mission();
 	mission_campaign_mission_over();	
 
-	if ( The_mission.flags[Mission::Mission_Flags::End_to_mainhall] ) {
+	if ( Campaign.next_mission == -1 || (The_mission.flags[Mission::Mission_Flags::End_to_mainhall]) ) {
 		gameseq_post_event( GS_EVENT_MAIN_MENU );
 	} else {
 		gameseq_post_event( GS_EVENT_START_GAME );