BUG/MEDIUM: mux_quic: fix memory leak of rx app_buf on stream free

BUG/MEDIUM: mux_quic: fix memory leak of rx app_buf on stream free

When freeing a QUIC stream (qcs), the receive application buffer
(qcs->rx.app_buf) was not released if it still contained data or
had been allocated. This led to a memory leak over time as streams
were opened and closed.

Fix this by explicitly freeing qcs->rx.app_buf via b_free() in
qcs_free() if its size is non-zero, and call offer_buffers() to
notify the buffer pool.

This should be backported as far as 2.6.