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>