MINOR: mux-quic: complete BUG_ON on TX flow-control enforcing

MINOR: mux-quic: complete BUG_ON on TX flow-control enforcing

TX flow-control enforcing is not straightforward : it requires the usage
of several counters at stream and connection level, in part due to the
difficult sending API between MUX and quic-conn layers.

To strengthen this part and ensures it behaves as expected, some
existing BUG_ON statements were adjusted and new one were added. This
should help to catch errors as early as possible, as in the case with
github issue #1738.