Release a potential reference to a db resource
Release a potential reference to a db resource

We had a problem in our app using the pdo_sqlsrv driver with PHP 7 that
manifested itself by a ODBC32.dll crash at the end of a PHP request
(see issue: https://github.com/Microsoft/msphpsql/issues/434).

It seems the dll tried to free a statement after the connection had been
disconnected.

We looked at our code and the ADOdb code to see if a reference to a
statement could imply that it would be garbage-collected after the PDO
disconnection.

We found that `ADODB_pdo#_query` returns a statement to
`ADOConnection#_Execute` which stores it in its `_queryID` property and
never empties it explicitly.

Setting it to `false` in `ADOConnection#Close` seems to fix the problem
in our app.

Fixes #379

ipso/ADOdb,ipso/ADOdb