client: fix off-by-one in wrapped pcap stats counter
When a libpcap stats counter (ps_recv/ps_drop/ps_ifdrop) wraps around UINT_MAX, get_psbl_wrapped_ctr() computed the delta as (UINT_MAX - old_val) + new_val, omitting the single increment that takes the counter from UINT_MAX through zero. Add the missing +1 so the reported delta matches the real number of increments.
