Run conn_cleanup on isccc_cc_towire failure in control_respond

Run conn_cleanup on isccc_cc_towire failure in control_respond

The bare return left conn->secret, conn->response, conn->request, and
conn->text pinned until the connection itself was torn down — every
other error in the function reaches conn_cleanup via goto, and the
success path falls into the same label, so the towire-failure return
was the lone outlier.  Send it through the existing cleanup path.

Assisted-by: Claude:claude-opus-4-7