fix: Linear to elbow conversion crash (#9556)

* Fix linear to elbow conversion

* Add invariant check

* Add dev invariant fix

* Add arrowhead

packages/excalidraw/components/ConvertElementTypePopup.tsx

@@ -564,7 +564,7 @@ export const convertElementTypes = (
               continue;
             }
             const fixedSegments: FixedSegment[] = [];
-            for (let i = 0; i < nextPoints.length - 1; i++) {
+            for (let i = 1; i < nextPoints.length - 2; i++) {
               fixedSegments.push({
                 start: nextPoints[i],
                 end: nextPoints[i + 1],
@@ -581,6 +581,7 @@ export const convertElementTypes = (
             );
             mutateElement(element, app.scene.getNonDeletedElementsMap(), {
               ...updates,
+              endArrowhead: "arrow",
             });
           } else {
             // if we're converting to non-elbow linear element, check if