[DCCP] ccid3: Check against too large p

This patch follows a suggestion by Ian McDonald and ensures that in
the current code the value of p can not exceed 100%.  Such a value is
illegal and would consequently cause a bug condition in tfrc_calc_x().

The receiver case is also tested, and a warning message is added.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>