Improve bwstat robustness

I got a program stuck into the loop inside bwstat_getdelay(). I believe that is
is due to the fact that I am using trickle in a multithread program and trickle
is not thread safe. Still, there 2 small things that can be done to improve
bwstat code:

1. Change pool,ncli and xent to detect overflow/underflow and quit the loop.
2. Initialize pts before inserting the new bwstat into the global list.

Signed-off-by: Olivier Langlois <olivier@olivierlanglois.net>