database/sql: ensure all driver Stmt are closed once

Previously  driver.Stmt could could be closed multiple times in
edge cases that drivers may not test for initially. Make their
job easier by ensuring the driver is only closed a single time.

Fixes #16019

Change-Id: I1e4777ef70697a849602e6ef9da73054a8feb4cd
Reviewed-on: https://go-review.googlesource.com/33352Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql_test.go

@@ -672,7 +672,7 @@ func TestStatementClose(t *testing.T) {
 		msg  string
 	}{
 		{&Stmt{stickyErr: want}, "stickyErr not propagated"},
-		{&Stmt{tx: &Tx{}, txsi: &driverStmt{&sync.Mutex{}, stubDriverStmt{want}}}, "driverStmt.Close() error not propagated"},
+		{&Stmt{tx: &Tx{}, txds: &driverStmt{Locker: &sync.Mutex{}, si: stubDriverStmt{want}}}, "driverStmt.Close() error not propagated"},
 	}
 	for _, test := range tests {
 		if err := test.stmt.Close(); err != want {