Refine assertion in caml_modify_local (#2244)

Author: mshinwell

ocaml/runtime/memory.c

@@ -358,8 +358,11 @@ CAMLexport int caml_is_stack (value v)
 CAMLexport void caml_modify_local (value obj, intnat i, value val)
 {
   if (Color_hd(Hd_val(obj)) == NOT_MARKABLE) {
-    /* This function should not be used on external values */
-    CAMLassert(caml_is_stack(obj));
+    /* This function should not be used on external values, but we have seen
+       some cases where it has been, in safe contexts where only immediate
+       values are involved. */
+    CAMLassert(caml_is_stack(obj)
+      || (!Is_block(val) && !Is_block(Field(obj, i))));
     Field(obj, i) = val;
   } else {
     caml_modify(&Field(obj, i), val);