Explicitly initialize stuff in ScenarioActivityConfigGUI

Undefined sanitizer's complaint:

../Source/Menus/ScenarioActivityConfigGUI.cpp:101:59: runtime error: load of value 190, which is not a valid value for type 'bool'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../Source/Menus/ScenarioActivityConfigGUI.cpp:101:59

...While working on this one, I discovered that C++
"default-initializes" class members, which leaves primitives and pointers
with undefined garbage values, but properly initializes classes.

And also the [Most vexing parse](https://en.wikipedia.org/wiki/Most_vexing_parse). Wow.

Author: Architector4

Source/Menus/ScenarioActivityConfigGUI.cpp

@@ -65,8 +65,6 @@ ScenarioActivityConfigGUI::ScenarioActivityConfigGUI(GUIControlManager* parentCo
 	m_CPULockLabel = dynamic_cast<GUILabel*>(m_GUIControlManager->GetControl("LabelCPUTeamLock"));
 	m_StartErrorLabel = dynamic_cast<GUILabel*>(m_GUIControlManager->GetControl("LabelStartError"));
 	m_StartGameButton = dynamic_cast<GUIButton*>(m_GUIControlManager->GetControl("ButtonStartGame"));
-
-	m_TechListFetched = false;
 }
 
 void ScenarioActivityConfigGUI::PopulateTechComboBoxes() {

Source/Menus/ScenarioActivityConfigGUI.h

@@ -65,16 +65,16 @@ namespace RTE {
 
 		GUIControlManager* m_GUIControlManager; //!< The GUIControlManager which holds all the GUIControls of this menu. Not owned by this.
 
-		const GameActivity* m_SelectedActivity; //!< The Activity this ScenarioActivityConfigGUI is configuring.
-		const GameActivity* m_PreviouslySelectedActivity; //!< The Activity this ScenarioActivityConfigGUI was configuring last, before it got was disabled.
-		Scene* m_SelectedScene; //!< The Scene the selected Activity will be using.
+		const GameActivity* m_SelectedActivity = nullptr; //!< The Activity this ScenarioActivityConfigGUI is configuring.
+		const GameActivity* m_PreviouslySelectedActivity = nullptr; //!< The Activity this ScenarioActivityConfigGUI was configuring last, before it got was disabled.
+		Scene* m_SelectedScene = nullptr; //!< The Scene the selected Activity will be using.
 		int m_LockedCPUTeam = Activity::Teams::NoTeam; //!< Which team the CPU is locked to, if any.
 
-		bool m_StartingGoldAdjustedManually {}; //!< Whether the player adjusted the starting gold, meaning it should stop automatically adjusting to the difficulty setting default starting gold where applicable.
+		bool m_StartingGoldAdjustedManually = false; //!< Whether the player adjusted the starting gold, meaning it should stop automatically adjusting to the difficulty setting default starting gold where applicable.
 
 		Timer m_StartGameButtonBlinkTimer; //!< Timer for blinking the start game button.
 
-		bool m_TechListFetched; //!< Whether the tech list was fetched and each team's ComboBox was populated with it, even if no valid tech modules were added.
+		bool m_TechListFetched = false; //!< Whether the tech list was fetched and each team's ComboBox was populated with it, even if no valid tech modules were added.
 
 		/// GUI elements that compose the Activity setup box.
 		GUICollectionBox* m_ActivityConfigBox;