Ensure that we validate the page header of the first page of a WAL file

whenever we start to read within that file.  The first page carries
extra identification information that really ought to be checked, but
as the code stood, this was only checked when we switched sequentially
into a new WAL file, or if by chance the starting checkpoint record was
within the first page.  This patch ensures that we will detect bogus
'long header' information before we start replaying the WAL sequence.