MINOR: mux-quic: use qcc_release in case of init failure

MINOR: mux-quic: use qcc_release in case of init failure

qmux_init() may fail at different stage. In this case, an error is
returned and QCC allocated element are freed. Previously, extra care was
taken using different label to only liberate already allocate elements.

This patch removes the multi label and uses qcc_release(). This will be
simpler to ensure a QCC is always properly freed. The only important
thing is to ensure that mandatory fields are properly initialized to
NULL or equivalent to be able to use qcc_release() safely.