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

…ebuggee

Use Code_val(v) instead of Field(v, 0) to make intent clearer.