Fix error handling of XLogReaderAllocate in case of OOM

Similarly to previous fix 9b8d4782, commit 2c03216d has switched
XLogReaderAllocate() to use a set of palloc calls instead of malloc,
causing any callers of this function to fail with an error instead of
receiving a NULL pointer in case of out-of-memory error. Fix this by
using palloc_extended with MCXT_ALLOC_NO_OOM that will safely return
NULL in case of an OOM.

Michael Paquier, slightly modified by me.