iproto: fix assertion on dropping of a new connection

We need to handle case of dropping new connection. When
net_send_greeting() is executed the connection can be closed due to
iproto_drop_connections() call.

Note that in the test the Tarantool crashes for another reason. Due to
access after sleep to the connection that is destroyed so its memory is
poisoned. Yet we visit net_send_greeting() too in the test with patch so
original issue is verified too. We also need to test that such
a connection is closed. This will be done in EE version.

Closes #9717

NO_DOC=bugfix