database/sql: check for nil Scan pointers

Return nice errors and don't panic.

Fixes issue 4859

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7383046
unicode: use new Scanner interface in table creation
Update norm and local/collate as well.

R=mpvl
CC=golang-dev
https://codereview.appspot.com/7395045