BUG/MAJOR: mux-quic: do not crash on connection error after glitch threshold

BUG/MAJOR: mux-quic: do not crash on connection error after glitch threshold

When a glitch reported by the HTTP/3 parser (SETTINGS/GOAWAY parsing or
QPACK decoding error) reaches tune.quic.{fe,be}.sec.glitches-threshold,
_qcc_report_glitch() sets a connection error before the caller also
calls qcc_set_error() with h3c->err. The BUG_ON() on double call then
kills the process, remotely triggerable by any client.

Make qcc_set_error() keep the first error and ignore subsequent calls.

Introduced by commit 0d35f8d91 ("MINOR: h3: report glitch on RFC
violation") in 3.0-dev12. Must be backported to all stable versions
since 3.0.