Fixes a case where the Stmt.Close() function in database/sql discards any error generated by the Close() function of the contained driverStmt.

Fixes #12798

Change-Id: I40384d6165856665b062d15a643e4ecc09d63fda
Reviewed-on: https://go-review.googlesource.com/15178Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>