MINOR: quic: ensure txbuf realloc is only performed on empty buffer

MINOR: quic: ensure txbuf realloc is only performed on empty buffer

QUIC application protocol layer has the ability to either allocate a
standard buffer or a smaller one. The latter is useful when only small
data are transferred to prevent consuming too much of the QUIC MUX
buffer window.

This operation is performed using qc_stream_buf_realloc(). Add a new
BUG_ON() in it to ensure no data is present in the buffer. Indeed, this
would cause to data loss, or even crash when trying to acknowledge data.

Note that for the moment qc_stream_buf_realloc() is only use for HTTP/3
headers transmission, and this usage is conform to the new BUG_ON. This
commit is thus not a bug fix, but only to strengthen the API.