Better handling of verify param id peername field

Better handling of verify param id peername field

Initialize pointers in param id by the book (explicit NULL assignment,
rather than just memset 0).

In x509_verify_param_zero() set peername to NULL after freeing it.

In x509_vfy.c's internal check_hosts(), avoid potential leak of
possibly already non-NULL peername.  This is only set when a check
succeeds, so don't need to do this repeatedly in the loop.

Reviewed-by: Richard Levitte <levitte@openssl.org>