Index build of a key with a key+payload >16bits, causes invalid key max length errors on read.

From 3.8.x the key building code has validated that key+payload is smaller than than the max key length.

The max key length in the header (for legacy reasons) is a 2 byte field, as a consequence a key+payload length >16bits, caused overflow and an invalid [small] result.

The net result was a spurious "key+payload .. exceeds max length .." error.