bug#41204: port-revealed count is initially > 30000

[Caleb Ristvedt]

(port-revealed (open-input-file "/dev/null"))
=> 32539

The little investigating I've done so far suggests that this is because
scm_i_fdes_to_port calls scm_gc_malloc_pointerless and initializes only
the 'fdes' and 'options' fields, leaving 'revealed' uninitialized. The
comment above scm_gc_malloc_pointerless does a good job of explaining
why this is an issue:

/* Allocate SIZE bytes of memory whose contents should not be scanned
   for pointers (useful, e.g., for strings).  Note though that this
   memory is *not* cleared; be sure to initialize it to prevent
   information leaks.  */

This issue appears to be present both in the 2.2 and 3.0 releases
packaged in guix.

- reepca