This replaces an unjustified BUG_ON(), which could get triggered under normal
conditions: X_calc can be 0 when p > 0. X would in this case be set to the
minimum, s/t_mbi. Its replacement avoids t_ipi = 0 (unbounded sending rate).

Thanks to Jordi, Victor and Xavier who reported this.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.uk>