BUG/MINOR: mux-quic: fix memleak on frames rejected by transport

BUG/MINOR: mux-quic: fix memleak on frames rejected by transport

When the MUX transfers a big amount of data to the client, the transport
layer may reject some of them because of the congestion controller
limit. Frames built by the MUX are thus dropped, even if streams transferred
data are kept in buffers for future new frames.

Thus, the MUX is required to free rejected frames. This fixes a memory
leak which may grow with important data transfers.

It should be backported to 2.6 after it has been tested and validated.