The second parameter for sql.putConn() (err) is always nil. As a result bad
connections are reused, even if the driver returns an driver.ErrBadConn.
Unsing a pointer to err instead achievs the desired behavior.
See http://code.google.com/p/go/issues/detail?id=3777 for more details.
Fixes #3777.

R=golang-dev, dave, bradfitz, jameshuachow, BlakeSGentry
CC=golang-dev
https://golang.org/cl/6348069