BUG/MINOR: xprt_qstrm: do not parse record length on read again

BUG/MINOR: xprt_qstrm: do not parse record length on read again

conn_recv_qstrm() may be called several times per connection if the read
data is too short and a truncated record is received.

Previously, record length was parsed every time the function is invoked.
However, this must only be performed if record length varint is
incomplete. Once read and parsed, data are removed from the buffer via
b_quic_dec_int(). Thus, next conn_recv_qstrm() run will reread an
invalid record length this time.

This patch fixes this by only parsing record length if <rxrlen> member
is null. Prior to it, parsing of QMux transport parameters would fail in
case of a first truncated read, which would prevent the connection
initialization.

No need to backport.