dccp ccid-3: Replace lazy BUG_ON with condition

The BUG_ON(w_tot == 0) only holds if there is no more than 1 loss interval in
the loss history. If there is only a single loss interval, the calc_i_mean()
routine need in fact not be called (RFC 3448, 6.3.1). 

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>