Fix GH-9309: Segfault when connection is used after imap_close()

We actually need to check whether `php_imap_object.imap_stream` is
`NULL` to detect that the connection has already been

Fix GH-9309: Segfault when connection is used after imap_close()

We actually need to check whether `php_imap_object.imap_stream` is
`NULL` to detect that the connection has already been closed.

Closes GH-9313.

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