(perl #132147) don't cache invalid pages

(perl #132147) don't cache invalid pages

When sdbm loads its page buffer from disk, in most cases it validates
the page and doesn't continue processing if it fails validation.

Unfortunately, in a few places it still marked the buffer as loaded
from that page, and later calls would then use that cached page,
causing a variety of problems, including buffer read overflows.

sdbm_firstkey() didn't validate the loaded page at all, it now does.

All places that validate the loaded page now on a failed validation:
  - invalidate the cached page (set pagbno to -1)
  - set the I/O error flag on the database object
  - set errno ($!) to EINVAL

The first ensures that later calls don't end up using an invalid cached
page.

The others allow the caller to check whether an error has occurred.