Fix read beyond buffer bug introduced by the split xlog.c patch.

Fix read beyond buffer bug introduced by the split xlog.c patch.

FinishWalRecovery() copied the valid part of the last WAL block into a
palloc'd buffer, and the code in StartupXLOG() copied it to the WAL
buffer. But the memcpy in StartupXLOG() copied a full 8kB block, not
just the valid part, i.e. it copied from beyond the end of the buffer.
The invalid part was cleared immediately afterwards, so as long as the
memory was allocated and didn't segfault, it didn't do any harm, but
it can definitely segfault.

Discussion: https://www.postgresql.org/message-id/efc12e32-5af2-3485-5b1d-5af9f707491a@iki.fi