pgsql/parser: fix response message length

Some backend messages can be the shortest pgsql length possible,
4 bytes, but the parser expectd all messages to be longer than that.

Related to
Bug #5524

rust/src/pgsql/parser.rs

@@ -727,7 +727,7 @@ pub fn parse_request(i: &[u8]) -> IResult<&[u8], PgsqlFEMessage> {
         b'X' => parse_terminate_message(i)?,
         _ => {
             let (i, identifier) = be_u8(i)?;
-            let (i, length) = verify(be_u32, |&x| x > PGSQL_LENGTH_FIELD)(i)?;
+            let (i, length) = verify(be_u32, |&x| x >= PGSQL_LENGTH_FIELD)(i)?;
             let (i, payload) = take(length - PGSQL_LENGTH_FIELD)(i)?;
             let unknown = PgsqlFEMessage::UnknownMessageType(RegularPacket {
                 identifier,