Fix randomly rejected form field submits (#8091)

If a formspec is submitted from a form fields handling
callback of another form (or "formspec shown from another
formspec"), the fields submitted for it can get
rejected by the form exploit mitigation subsystem with a
message like "'zorman2000' submitted formspec
('formspec_error:form2') but server hasn't sent formspec to
client, possible exploitation attempt" being sent to logs.
This was already reported as #7374 and a change was made
that fixed the simple testcase included with that bug
report but the bug still kept lurking around and popping
out in more complicated scenarios like the advtrains TSS
route programming UI.

Deep investigation of the problem revealed that this
sequence of events is entirely possible and leads to the
bug:

  1. Server: show form1
  2. Client *shows form1*
  3. Client: submits form1
  4. Server: show form2
  5. Client: says form1 closed
  6. Client *shows form2*
  7. Client: submits form2

What happens inside the code is that when the server in
step 4 sends form2, the registry of opened forms is
updated to reflect the fact that form2 is now the valid
form for the client to submit. Then when in step 5 client
says "form1 was closed", the exploit mitigation subsystem
code deletes the registry entry for the client without
bothering to check whether the form client says was
closed just now is indeed the form that is recorded in
that entry as the valid form. Then later, in step 7 the
client tries to submit its valid form fields, these will
be rejected because the entry is missing.

It turns out the procedure where the broken code resides
already gets the form name so a simple "if" around the
offending piece of code fixes the whole thing. And
advtrains TSS agrees with that.

src/server.cpp

@@ -1577,7 +1577,11 @@ void Server::SendShowFormspecMessage(session_t peer_id, const std::string &forms
 	NetworkPacket pkt(TOCLIENT_SHOW_FORMSPEC, 0 , peer_id);
 	if (formspec.empty()){
 		//the client should close the formspec
+		//but make sure there wasn't another one open in meantime
+		const auto it = m_formspec_state_data.find(peer_id);
+		if (it != m_formspec_state_data.end() && it->second == formname) {
 			m_formspec_state_data.erase(peer_id);
+		}
 		pkt.putLongString("");
 	} else {
 		m_formspec_state_data[peer_id] = formname;