Fix panic when scrolling through completion popup (#1260)

* fix(completion_popup): Fixes #1256

* Update helix-term/src/ui/completion.rs

Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>

helix-term/src/ui/completion.rs

@@ -328,8 +328,8 @@ impl Component for Completion {
                 let y = popup_y;
 
                 if let Some((rel_width, rel_height)) = markdown_doc.required_size((width, height)) {
-                    width = rel_width;
+                    width = rel_width.min(width);
-                    height = rel_height;
+                    height = rel_height.min(height);
                 }
                 Rect::new(x, y, width, height)
             } else {