fixed the way of iterating over the range of map.

Fixed the following errors:
1. Request(0) causes a dead loop when the map is full and map.last == BEGIN.
2. When map.last is the only available port (or ip), Request(0) returns ErrAllPortsAllocated (or ErrNoAvailableIPs). Exception is when map.last == BEGIN.

Signed-off-by: shuai-z <zs.broccoli@gmail.com>