QUIC: fixed in-flight bytes accounting. Initially, frames are genereated and stored in ctx->frames. Next, ngx_quic_output() collects frames to be sent in in ctx->sending. On failure, ngx_quic_revert_sned() returns frames into ctx->frames. On success, the ngx_quic_commit_send() moves ack-eliciting frames into ctx->sent and frees non-ack-eliciting frames. This function also updates in-flight bytes counter, so only actually sent frames are accounted. The counter is decremented in the following cases: - acknowledgment is received - packet was declared lost - we are discarding context completely In each of this cases frame is removed from ctx->sent queue and in-flight counter is accordingly decremented. The patch fixes the case of discarding context - only removing frames from ctx->sent must be followed by in-flight bytes counter decrement, otherwise cg->in_flight could experience type underflow. The issue appeared in b1676cd64dc9.