Fix mfc overflow check and uninitialized variable.

Fix mfc overflow check and uninitialized variable.

The check borrowed from mx_alloc_memory() works because it is
incremented 25 at a time.  I don't believe it will work for the direct
set case used in imap_alloc_msn_index().  Instead, use a more
conservative check.

In imap_read_headers(), make sure mfhrc is initialized.  It would be
tested without being set if imap_cmd_step() returned OK right away.