database/sql: clone data for named []byte types

database/sql: clone data for named []byte types

Previously named byte types like json.RawMessage could get dirty
database memory from a call to Scan. These types would activate a
code path that didn't clone the byte data coming from the database
before assigning it. Another thread could then overwrite the byte
array in src, which has unexpected consequences.

Originally reported by Jason Moiron; the patch and test are his
suggestions. Fixes #13905.

Change-Id: Iacfef61cbc9dd51c8fccef9b2b9d9544c77dd0e0
Reviewed-on: https://go-review.googlesource.com/22393
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>