is an overlap between BLOB pointers and the modification log or the
zlib stream.

page_zip_decompress_clust_ext(): Remove the improper check.  The
d_stream->avail_in cannot be decremented here, because we do not know
at this point if the record is deleted.  No space is reserved for the
BLOB pointers in deleted records.

page_zip_decompress_clust(): Check for the overlap here, right before
copying the BLOB pointers.

page_zip_decompress_clust(): Also check that the target column is long
enough, and return FALSE instead of ut_ad() failure.