Fix T78900: Single vertex sliding crashes

As we can see in `initVertSlide_ex`, `sld` can be `NULL`.
`sld` is dereferenced, but can still be `NULL`.

source/blender/editors/transform/transform_mode_vert_slide.c

@@ -500,6 +500,10 @@ static void doVertSlide(TransInfo *t, float perc)
 
   FOREACH_TRANS_DATA_CONTAINER (t, tc) {
     VertSlideData *sld = tc->custom.mode.data;
+    if (sld == NULL) {
+      continue;
+    }
+
     TransDataVertSlideVert *svlist = sld->sv, *sv;
     int i;