Prevent the game from still attempting to save replays during a game exit

Resolves SIGSEGV no. 8

src/g_demo.c

@@ -4364,7 +4364,7 @@ boolean G_CheckDemoStatus(void)
 		return true;
 	}
 
-	if (demo.recording && (modeattacking || demo.savemode != DSM_NOTSAVING))
+	if (demo.recording && (modeattacking || demo.savemode != DSM_NOTSAVING) && (!demo.gamequit))
 	{
 		G_SaveDemo();
 		return true;

src/g_demo.h

@@ -59,6 +59,7 @@ struct demovars_s {
 	} savemode;
 
 	boolean freecam;
+	boolean gamequit; // Game is exiting.
 
 	const savebuffer_t *buffer; // debug, valid only if recording or playback
 };

src/sdl/i_system.cpp

@@ -1936,7 +1936,13 @@ void I_Quit(void)
 	//        but sometimes we forget and use 'F10'.. so save here too.
 
 	if (demo.recording)
+	{
+		// SIGSEGV #8 is caused by the game still trying to save to an empty pointer.
+		// Prevent that by doing... this!
+		demo.gamequit = true;
+
 		G_CheckDemoStatus();
+	}
 	if (metalrecording)
 		G_StopMetalRecording(false);