- fix crash on dying when player class has no death states

https://forum.zdoom.org/viewtopic.php?t=60815
(cherry picked from commit 7009755c52)

src/p_interaction.cpp

@@ -719,7 +719,9 @@ void AActor::Die (AActor *source, AActor *inflictor, int dmgflags)
 				tics = 1;
 		}
 	}
-	else
+	// The following condition is needed to avoid crash when player class has no death states
+	// Instance of player pawn will be garbage collected on reloading of level
+	else if (player == nullptr)
 	{
 		Destroy();
 	}