some decompression functions.

page_zip_apply_log_ext(), page_zip_apply_log(): Call page_zip_fail()
with appropriate diagnostics before returning NULL.

page_zip_decompress_node_ptrs(), page_zip_decompress_sec(),
page_zip_decompress_clust(): When detecting that the zlib stream
followed by the modification log overlaps the trailer, do not
let an assertion fail, but invoke page_zip_fail() and return FALSE.
Corrupt data should never lead into assertion failures in decompression
functions.