Clear list rings when loading a new formspec

Fixes a bug where the old list ring remained when a new formspec
was displayed over the old one. This created the list-ring of the new formspec
to be partly ignored.

Thanks to @VanessaE to report the bug, and @DonBatman to produce the code that
exposed it.

src/guiFormSpecMenu.cpp

@@ -1880,6 +1880,7 @@ void GUIFormSpecMenu::regenerateGui(v2u32 screensize)
 	m_fields.clear();
 	m_boxes.clear();
 	m_tooltips.clear();
+	m_inventory_rings.clear();
 
 	// Set default values (fits old formspec values)
 	m_bgcolor = video::SColor(140,0,0,0);