Ensure failed request buffers are cleaned up

When using `NewBufferedReadCloser`, it's normally the client's
responsibility to `Close()` the returned buffer. However, if there's an
error when initially populating the buffer, we don't return the failed
buffer to the client, and we weren't closing it ourselves. This leads to
stale buffers being left behind.

Instead, if we can't return a properly populated buffer, we should close
and discard it before returning nil.

internal/server/buffer.go

@@ -35,6 +35,7 @@ func NewBufferedReadCloser(r io.ReadCloser, maxBytes, maxMemBytes int64) (io.Rea
 
 	_, err := io.Copy(buf, r)
 	if err != nil {
+		buf.Close()
 		return nil, err
 	}