Fix a crash in :inspect when switching from page 2 to a different item

Switching to a different item cleared the data, but didn't clear the
cache of where the page breaks are located. The subsequent call to
`get_data_paged(int which_page)` has a bounds check for the page number,
so it's sufficient to just clear the page breaks, which is already ok
as it resets the object to the same state as a newly-created instance.

Forward-ported but currently untestable in master, as the page-selection
buttons are only shown when there's a large amount of text, which triggers
the known regression (the credits-crash bug).

(cherry picked from commit 7cb5f09bcf)

changelog_entries/crash_inspect_page.md

@@ -0,0 +1,2 @@
+ ### Miscellaneous and Bug Fixes
+   * Fix a crash in the `:inspect` window when pagination is used (issue #7851).

src/gui/dialogs/gamestate_inspector.cpp

@@ -92,6 +92,7 @@ public:
 	void clear_data()
 	{
 		data.clear();
+		pages.clear();
 	}
 
 	void set_data(const std::string& new_data)