Wrong unmarshaling of function pointers in debugger mode

In debugger mode, closures sent by the debuggee have their code pointer
modified so that it points to the `function_placeholder` code in
debugger/main.ml.

This modification was performed incorrectly: the whole closure for
`function_placeholder` was used as the code pointer for the
unmarshalled closure.

This commit implements the correct operation: the code pointer from
`function_placeholder` is used as code pointer for the unmarshaled
closure.

Fixes: ocaml#9214

runtime/intern.c

@@ -511,7 +511,8 @@ static void intern_rec(value *dest)
           const value * function_placeholder =
             caml_named_value ("Debugger.function_placeholder");
           if (function_placeholder != NULL) {
-            v = *function_placeholder;
+            /* Use the code pointer from the "placeholder" function */
+            v = Field(*function_placeholder, 0);
           } else {
             intern_cleanup();
             intern_bad_code_pointer(digest);