QUIC: fixed output context restoring.

The cd8018bc81a5 fixed unintended send of non-padded initial packets,
but failed to restore context properly: only processed contexts need
to be restored.  As a consequence, a packet number could be restored
from uninitialized value.

src/event/quic/ngx_event_quic_output.c

@@ -165,7 +165,7 @@ ngx_quic_create_datagrams(ngx_connection_t *c)
             if (min > len) {
                 /* padding can't be applied - avoid sending the packet */
 
-                for (i = 0; i < NGX_QUIC_SEND_CTX_LAST; i++) {
+                while (i-- > 0) {
                     ctx = &qc->send_ctx[i];
                     ngx_quic_revert_send(c, ctx, preserved_pnum[i]);
                 }