recv_ring::copy_if_needed(): If the record wraps around the
memory-mapped ib_logfile0, do copy it also if len==0
(the record consists only of a header, like FREE_PAGE and INIT_PAGE
records do).

recv_sys_t::parse(): Invoke recv_ring::copy_if_needed() for INIT_PAGE
and FREE_PAGE records, so that if these records wrap around the
memory-mapped ib_logfile0, they will be correctly copied to
recv_sys.pages.

Together with commit 0d175968 (MDEV-31354)
this fixes occasional failures of the test innodb.recovery_memory.