Do not clear txn state inside nested statement

If a connection does e.g. CREATE TABLE, it will start a "child statement"
to reparse the schema. That statement does not start its own transaction,
and so should not try to end the existing one either.

We had a logic bug where these steps would happen:

- `CREATE TABLE` executed successfully
- pread fault happens inside `ParseSchema` child stmt
- `handle_program_error()` is called
- `pager.end_tx()` returns immediately because `is_nested_stmt` is true
  and we correctly no-op it.
- however, crucially: `handle_program_error()` then sets tx state to None
- parent statement now catches error from nested stmt and calls
  `handle_program_error()`, which calls `pager.end_tx()` again, and since
  txn state is None, when it calls `rollback()` we panic on the assertion
 `"dirty pages should be empty for read txn"`

Solution:

Do not do _any_ error processing in `handle_program_error()` inside a nested
stmt. This means that the parent write txn is still active when it processes
the error from the child and we avoid this panic.

core/vdbe/mod.rs

@@ -857,6 +857,10 @@ pub fn handle_program_error(
     connection: &Connection,
     err: &LimboError,
 ) -> Result<()> {
+    if connection.is_nested_stmt.get() {
+        // Errors from nested statements are handled by the parent statement.
+        return Ok(());
+    }
     match err {
         // Transaction errors, e.g. trying to start a nested transaction, do not cause a rollback.
         LimboError::TxError(_) => {}