Fix #10438: [GS] Validate story page element type for ScriptStoryPage::NewElement (#11888)

Fixes a crash that happens if an invalid StoryPageElementType is passed to ScriptStoryPage::NewElement.

Adds an enforced precondition that tests the validity of StoryPageElementType.

Adds GSStoryPage::IsValidStoryPageElementType to the API.

src/script/api/game_changelog.hpp

@@ -81,6 +81,7 @@
  * \li GSGoal::SetDestination
  * \li GSIndustry::GetProductionLevel
  * \li GSIndustry::SetProductionLevel
+ * \li GSStoryPage::IsValidStoryPageElementType
  *
  * API removals:
  * \li GSError::ERR_PRECONDITION_TOO_MANY_PARAMETERS, that error is never returned anymore.

src/script/api/script_story_page.cpp

@@ -38,6 +38,11 @@ static inline bool StoryPageElementTypeRequiresText(StoryPageElementType type)
 	return ::StoryPageElement::IsValidID(story_page_element_id);
 }
 
+/* static */ bool ScriptStoryPage::IsValidStoryPageElementType(StoryPageElementType type)
+{
+	return type == SPET_TEXT || type == SPET_LOCATION || type == SPET_GOAL || type == SPET_BUTTON_PUSH || type == SPET_BUTTON_TILE || type == SPET_BUTTON_VEHICLE;
+}
+
 /* static */ ScriptStoryPage::StoryPageID ScriptStoryPage::New(ScriptCompany::CompanyID company, Text *title)
 {
 	CCountedPtr<Text> counter(title);
@@ -63,6 +68,7 @@ static inline bool StoryPageElementTypeRequiresText(StoryPageElementType type)
 
 	EnforceDeityMode(STORY_PAGE_ELEMENT_INVALID);
 	EnforcePrecondition(STORY_PAGE_ELEMENT_INVALID, IsValidStoryPage(story_page_id));
+	EnforcePrecondition(STORY_PAGE_ELEMENT_INVALID, IsValidStoryPageElementType(type));
 	std::string encoded_text;
 	if (StoryPageElementTypeRequiresText(btype)) {
 		EnforcePrecondition(STORY_PAGE_ELEMENT_INVALID, text != nullptr);

src/script/api/script_story_page.hpp

@@ -179,6 +179,13 @@ public:
 	 */
 	static bool IsValidStoryPageElement(StoryPageElementID story_page_element_id);
 
+	/**
+	 * Check whether this is a valid story page element type.
+	 * @param type The StoryPageElementType to check.
+	 * @return True if and only if this story page element type is valid.
+	 */
+	static bool IsValidStoryPageElementType(StoryPageElementType type);
+
 	/**
 	 * Create a new story page.
 	 * @param company The company to create the story page for, or ScriptCompany::COMPANY_INVALID for all.
@@ -202,6 +209,7 @@ public:
 	 * @return The new StoryPageElementID, or STORY_PAGE_ELEMENT_INVALID if it failed.
 	 * @pre ScriptCompanyMode::IsDeity().
 	 * @pre IsValidStoryPage(story_page).
+	 * @pre IsValidStoryPageElementType(type).
 	 * @pre (type != SPET_TEXT && type != SPET_LOCATION) || (text != null && len(text) != 0).
 	 * @pre type != SPET_LOCATION || ScriptMap::IsValidTile(reference).
 	 * @pre type != SPET_GOAL || ScriptGoal::IsValidGoal(reference).