Fix race in ExecContext

When the context is cancelled, an interrupt should only be made if the
operation is still ongoing.

sqlite3_go18_test.go

@@ -134,3 +134,24 @@ func TestShortTimeout(t *testing.T) {
 		t.Fatal(ctx.Err())
 	}
 }
+
+func TestExecCancel(t *testing.T) {
+	db, err := sql.Open("sqlite3", ":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	if _, err = db.Exec("create table foo (id integer primary key)"); err != nil {
+		t.Fatal(err)
+	}
+
+	for n := 0; n < 100; n++ {
+		ctx, cancel := context.WithCancel(context.Background())
+		_, err = db.ExecContext(ctx, "insert into foo (id) values (?)", n)
+		cancel()
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}