database/sql: Check errors in QueryRow.Scan

database/sql: Check errors in QueryRow.Scan

The previous coding did not correctly check for errors from the driver's
Next() or Close(), which could mask genuine errors from the database, as
witnessed in issue #6651.

Even after this change errors from Close() will be ignored if the query
returned no rows (as Rows.Next will have closed the handle already), but it
is a lot easier for the drivers to guard against that.

Fixes #6651.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/41590043