database/sql: fix possible context leak in test

Fixes #17560

Change-Id: I96fcdec87220391ef5432571b5c090b5be27491a
Reviewed-on: https://go-review.googlesource.com/31771
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/database/sql/sql_test.go b/src/database/sql/sql_test.go
index 228a3f2..34bbc66 100644
--- a/src/database/sql/sql_test.go
+++ b/src/database/sql/sql_test.go
@@ -267,6 +267,7 @@
 	prepares0 := numPrepares(t, db)
 
 	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 
 	rows, err := db.QueryContext(ctx, "SELECT|people|age,name|")
 	if err != nil {