BUG/MINOR: trace/quic_frame: use buf, not trace_buf in chunk_frm_appendf()

BUG/MINOR: trace/quic_frame: use buf, not trace_buf in chunk_frm_appendf()

The function takes a buffer in argument which is the target buffer. The
first calls properly use it but the subsequent ones, probably due to
reused/moved code, directly write into &trace_buf, thus ignoring the
buf argument. Fortunately all call places pass &trace_buf for buf, so
it currently has no impact but could possibly change.

No backport is needed, but it doesn't hurt to backport it if it helps.