Users with 'update own <type> content' but without 'create <type> content' are allowed to create content (OgSelectionHandler)

To Reproduce

Login as user that has OG permission 'update own page content' but lacks 'create page content' in a group. Go to create page form. Said group is listed in group audience field. Select that group, fill out the form, and create page content.

Expected behavior

Said group will not be listed in group audience field and creation of page in group will not be allowed.

Reason + patch

The access check added in #1541672: Don't check "create" access in OgSelectionHandler::buildEntityFieldQuery() to OgSelectionHandler only checks that $node exists or $node->uid == $user->uid, but does not check $node->nid. At some point in the form render process, a stub $node is created, so these checks succeed even though the content does not yet exist.

This patch checks $node->nid explicitly. Patch is against 7.x-2.x.