MDEV-23334 Crash in rec_get_nth_cfield()/rec_offs_validate()

rec_get_nth_cfield(): Remove a bogus debug assertion.
The function may be invoked by innobase_rec_to_mysql()
for reporting a duplicate key error during CREATE UNIQUE INDEX
or ALTER TABLE...ADD UNIQUE KEY, and in that case the record
will be missing the 5-byte or 6-byte fixed header.

It turns out that in every other code path leading to
rec_get_nth_cfield() we either invoked rec_get_offsets()
ourselves or asserted rec_offs_validate(). So, we can
safely remove the assertion and make debug builds
smaller and faster.

storage/innobase/include/rem0rec.h

@@ -829,7 +829,11 @@ rec_get_nth_cfield(
 	ulint			n,
 	ulint*			len)
 {
-	ut_ad(rec_offs_validate(rec, index, offsets));
+	/* Because this function may be invoked by innobase_rec_to_mysql()
+	for reporting a duplicate key during ALTER TABLE or
+	CREATE UNIQUE INDEX, and in that case the rec omit the fixed-size
+	header of 5 or 6 bytes, the check
+	rec_offs_validate(rec, index, offsets) must be avoided here. */
 	if (!rec_offs_nth_default(offsets, n)) {
 		return rec_get_nth_field(rec, offsets, n, len);
 	}