Missing initialization in caml_obj_reachable_words

A marshaling operation can leave `extern_flags` with the `NO_SHARING`
bit set.  In this context, `caml_obj_reachable_words` calls
`extern_init_position_table`, which does nothing, then proceeds to
access the position table, causing a crash.

The solution is trivial: initialize `extern_flags` before calling
`extern_init_position_table`.

First reported at ocaml-multicore/ocaml-multicore#824

runtime/extern.c

@@ -1136,6 +1136,7 @@ CAMLprim value caml_obj_reachable_words(value v)
   uintnat pos;
 
   obj_counter = 0;
+  extern_flags = 0;
   extern_init_position_table();
   sp = extern_stack;
   size = 0;