IMPORT TABLESPACE must validate record offsets and BLOB pointer length

ALTER TABLE ... IMPORT TABLESPACE reads every page of the .ibd file
being imported and, for the clustered index, rewrites each record's
DB_TRX_ID/DB_ROLL_PTR system columns and any BLOB external reference
it finds (PageConverter::update_records() and adjust_cluster_index_blob_column()).
The imported page's internal checksum is verified before this step,
but nothing checks that the record positions and record offsets computed
from the page content, nor the length field inside a BLOB external
reference, are actually sane. They are trusted as if they came from
a page InnoDB itself had written.

Two concrete problems were found:

1. Unvalidated record offsets. The record "next" pointer chaining
records together on a page is only range-checked by a debug-only
assertion. In a release build a corrupted pointer can make the
iterator land on an arbitrary byte within the page, which is then
decoded as if it were a real record's field count/offsets

2. Unvalidated BLOB pointer length. A BLOB external reference's
8-byte length field (BTR_EXTERN_LEN) is trusted outright.
btr_copy_externally_stored_field() only consults the low 32 bits of
this field, so a corrupted high dword is completely invisible to
ordinary reads

This is created to port mysql bug#39295605 patch