Fix up lib/postgres/postgre-pr/message.rb and its self.read function so that it will handle cases where stream reads might not return any bytes at all, resulting in length being nil

lib/postgres/postgres-pr/message.rb

@@ -46,7 +46,7 @@ class Message
     type = stream.read_exactly_n_bytes(1) unless startup
     length = stream.read_exactly_n_bytes(4).to_s.unpack('N').first  # FIXME: length should be signed, not unsigned
 
-    raise ParseError unless length >= 4
+    raise ParseError unless (!length.nil? && length >= 4)
 
     # initialize buffer
     buffer = Buffer.of_size(startup ? length : 1+length)