Have encapsulation.ReadData return an error when the buffer is short.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/154#note_2919109

Still ignoring the io.ErrShortBuffer at the callers, which retains
current behavior.

server/lib/http.go

@@ -176,6 +176,9 @@ func (handler *httpHandler) turbotunnelMode(conn net.Conn, addr net.Addr) error
 		var p [2048]byte
 		for {
 			n, err := encapsulation.ReadData(conn, p[:])
+			if err == io.ErrShortBuffer {
+				err = nil
+			}
 			if err != nil {
 				return
 			}