UI: Clear fade to black source

If the fade to black was active and the user switched scene collections,
or exited studio mode, the source wouldn't be cleared, causing the
transition not to work properly when used again.

UI/window-basic-main-transitions.cpp

@@ -1689,6 +1689,7 @@ void OBSBasic::SetPreviewProgramMode(bool enabled)
 
 		programScene = nullptr;
 		swapScene = nullptr;
+		prevFTBSource = nullptr;
 
 		for (QuickTransition &qt : quickTransitions)
 			qt.button = nullptr;

UI/window-basic-main.cpp

@@ -4368,6 +4368,7 @@ void OBSBasic::ClearSceneData()
 	lastScene = nullptr;
 	swapScene = nullptr;
 	programScene = nullptr;
+	prevFTBSource = nullptr;
 
 	copyStrings.clear();
 	copyFiltersString = nullptr;