Tolerate that the cursor might not be visible

Sometimes, like if the window is not high enough, we might not be
able to display any buffer lines, hence not have the cursor visible.

Fixes #1502

src/input_handler.cc

@@ -41,7 +41,8 @@ public:
 
     virtual std::pair<CursorMode, DisplayCoord> get_cursor_info() const
     {
-        DisplayCoord coord = *context().window().display_position(context().selections().main().cursor());
+        const auto cursor = context().selections().main().cursor();
+        auto coord = context().window().display_position(cursor).value_or(DisplayCoord{});
         return {CursorMode::Buffer, coord};
     }