database/sql: guard against driver.Stmt.Close panics

Do not retain a lock when driver.Stmt.Close panic as the rest
of the sql package ensures.

Updates #16019

Change-Id: Idc7ea9258ae23f491e79cce3efc365684a708428
Reviewed-on: https://go-review.googlesource.com/33328
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -408,17 +408,19 @@ func (dc *driverConn) Close() error {
 }
 
 func (dc *driverConn) finalClose() error {
-	dc.Lock()
-
-	for si := range dc.openStmt {
-		si.Close()
-	}
-	dc.openStmt = nil
-
-	err := dc.ci.Close()
-	dc.ci = nil
-	dc.finalClosed = true
-	dc.Unlock()
+	var err error
+	withLock(dc, func() {
+		defer func() { // In case si.Close panics.
+			dc.openStmt = nil
+			dc.finalClosed = true
+			err = dc.ci.Close()
+			dc.ci = nil
+		}()
+
+		for si := range dc.openStmt {
+			si.Close()
+		}
+	})
 
 	dc.db.mu.Lock()
 	dc.db.numOpen--