HTTP/3: protection from recursion during connection reuse. When draining a connection associated with an HTTP/3 stream, calling ngx_http_v3_send_cancel_stream() might result in an attempt to obtain a connection for the decoder stream. This in turn will trigger draining of the very same connection. Depending on the client settings, this might either lead to stack overflow or will end up in decoder stream creation error and destroying the connection at some point, potentially resulting in use-after-free on stack. Fix is to make sure that connection reuse is disabled in ngx_http_v3_reset_stream(), so the recursion in question won't happen regardless of what called functions do.