BUG/MINOR: h3: close connection on response start-line alloc failure

BUG/MINOR: h3: close connection on response start-line alloc failure

On QUIC backend side, correct the error handling when HTX start-line
cannot be allocated during HTTP/3 response parsing.

Previously, a stream error would be reported with H3_MESSAGE_ERROR. Now,
h3_resp_headers_to_htx() only returns a default negative value without
any explicit error code. This results in a connection level error using
H3_INTERNAL_ERROR, generated at the end of h3_rcv_buf().

This pattern is already used on request parsing. It is necessary to
close the connection asap on memory exhaustion while differentiating
between a peer protocol violation and an haproxy internal error.

This must be backported up to 3.3.