Fix T66567: Weight painting crashes with specific mesh

Seems the deform group index and deform vertices went out of sync somehow.

Added extra NULL pointer check, which seems to be safe and matches checks
in other places in the neighbourhood.

source/blender/blenkernel/intern/armature.c

@@ -1370,7 +1370,11 @@ static void armature_vert_task(void *__restrict userdata,
   if (use_dverts || armature_def_nr != -1) {
     if (data->mesh) {
       BLI_assert(i < data->mesh->totvert);
-      dvert = data->mesh->dvert + i;
+      if (data->mesh->dvert != NULL) {
+        dvert = data->mesh->dvert + i;
+      } else {
+        dvert = NULL;
+      }
     }
     else if (data->dverts && i < data->target_totvert) {
       dvert = data->dverts + i;