Fix #140962: Vertex Slide with Proportional Editing crashes Blender

Use a separate index for indexing into `Array<TransDataVertSlideVert>
r_sv`, as that only contains the selected items (and not all items
considered for proportional editing).

Since the introduction of the sorted index map (df6d345bb4) the
selected "transform items" are simply visited first, but not sorted to
the front of the array. This means that these array indices cannot be
used to index into other arrays that only contain sorted data, because
they will overflow.

Ref !140976

source/blender/editors/transform/transform_convert_mesh.cc

@@ -2166,6 +2166,7 @@ Array<TransDataVertSlideVert> transform_mesh_vert_slide_data_create(
   Array<TransDataVertSlideVert> r_sv(td_selected_len);
 
   r_loc_dst_buffer.reserve(r_sv.size() * 4);
+  int r_sv_index = 0;
   tc->foreach_index_selected([&](const int i) {
     TransData *td = &tc->data[i];
     const int size_prev = r_loc_dst_buffer.size();
@@ -2186,13 +2187,15 @@ Array<TransDataVertSlideVert> transform_mesh_vert_slide_data_create(
       }
     }
 
-    TransDataVertSlideVert &sv = r_sv[i];
+    TransDataVertSlideVert &sv = r_sv[r_sv_index];
     sv.td = &tc->data[i];
     /* The buffer address may change as the vector is resized. Avoid setting #Span. */
     // sv.targets = r_loc_dst_buffer.as_span().drop_front(size_prev);
 
     /* Store the buffer size temporarily in `target_curr`. */
     sv.co_link_curr = r_loc_dst_buffer.size() - size_prev;
+
+    r_sv_index++;
   });
 
   int start = 0;