database/sql: fix memory leaks in Stmt.removeClosedStmtLocked

Zero out elements before shrinking the slice to avoid memory leaks.

Fixes golang#66410

Change-Id: I8f64c21455761f7f7c8b6fee0b6450b98f691d91
GitHub-Last-Rev: b15586e
GitHub-Pull-Request: golang#66419
Reviewed-on: https://go-review.googlesource.com/c/go/+/572956
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>

src/database/sql/sql.go

@@ -2684,6 +2684,8 @@ func (s *Stmt) removeClosedStmtLocked() {
 	for i := 0; i < len(s.css); i++ {
 		if s.css[i].dc.dbmuClosed {
 			s.css[i] = s.css[len(s.css)-1]
+			// Zero out the last element (for GC) before shrinking the slice.
+			s.css[len(s.css)-1] = connStmt{}
 			s.css = s.css[:len(s.css)-1]
 			i--
 		}