Certain directories in `/proc` can cause the `ReadDir`
iterator to loop indefinitely. We get an error code (22) when
calling libc's `readdir_r` on these directories, but `entry_ptr`
is `NULL` at the same time, signalling the end of the directory
stream.

This change introduces an internal state to the iterator such
that the `Some(Err(..))` value will only be returned once when
calling `next`. Subsequent calls will return `None`.

fixes #50619