Fix #107020: crash when canceling Sky Resize with mesh symmetry

Like `t->data` use calloc to `tc->data_mirror`.

This way you make sure that all values are properly initialized.

source/blender/editors/transform/transform_convert_mesh_skin.c

@@ -159,7 +159,7 @@ static void createTransMeshSkin(bContext *UNUSED(C), TransInfo *t)
 
       if (mirror_data.vert_map) {
         tc->data_mirror_len = mirror_data.mirror_elem_len;
-        tc->data_mirror = MEM_mallocN(mirror_data.mirror_elem_len * sizeof(*tc->data_mirror),
+        tc->data_mirror = MEM_callocN(mirror_data.mirror_elem_len * sizeof(*tc->data_mirror),
                                       __func__);
 
         BM_ITER_MESH_INDEX (eve, &iter, bm, BM_VERTS_OF_MESH, a) {