BUG/MEDIUM: mux-h2: Don't send RST_STREAM frame for streams with no ID

BUG/MEDIUM: mux-h2: Don't send RST_STREAM frame for streams with no ID

On server side, the H2 stream is first created with an unassigned ID (ID ==
0). Its ID is assigned when the request is emitted, before formatting the
HEADERS frame. However, the session may be aborted during that stage. We
must take care to not emit RST_STREAM frame for this stream, because it does
not exist yet for the server.

It is especially important to do so because, depending on the timing, it may
also happens before the H2 PREFACE was sent.

This patch must be backported to all stable versions. It is related to issue