Only check error length during startup, limit response size for some types

[arp242]

#1248 is not entirely correct, as it should only apply on startup messages, as that check is done in PQconnectPoll():

if (beresp == PqMsg_ErrorResponse && (msgLength < 8 || msgLength > MAX_ERRLEN)

We can re-use the txnStatus field to check if we're in the connection phase.

In addition, in pqParseInput3(), it does limit the message length, but only for some respose types:

#define VALID_LONG_MESSAGE_TYPE(id) \
	((id) == PqMsg_CopyData || \
	(id) == PqMsg_DataRow || \
	(id) == PqMsg_ErrorResponse || \
	(id) == PqMsg_FunctionCallResponse || \
	(id) == PqMsg_NoticeResponse || \
	(id) == PqMsg_NotificationResponse || \
	(id) == PqMsg_RowDescription)

if (msgLength > 30000 && !VALID_LONG_MESSAGE_TYPE(id))

So check for that as well.

Fixes #1324
Closes #1326