Checking if unsigned variable is less than zero.

[ZanyXDev]

https://github.com/tass-belgium/picotcp/blob/master/modules/pico_olsr.c#L324
unsigned variable 'sublist->time_left'
if (sublist->time_left <= 0) but

struct olsr_route_entry { struct olsr_route_entry *next; ...
uint32_t time_left;
... }