BUG/MINOR: mux-quic: properly handle buf alloc failure

BUG/MINOR: mux-quic: properly handle buf alloc failure

A convenience function qc_get_buf() is implemented to centralize buffer
allocation on MUX and H3 layers. However, allocation failure was not
handled properly with a BUG_ON() statement.

Replace this by proper error management. On emission, streams is
temporarily skip over until the next qc_send() invocation. On reception,
H3 uses this function for HTX conversion; on alloc failure the
connection will be closed with QUIC internal error code.

This must be backported up to 2.6.