dhcp-server: T1745: bugfix corner case on static-assignments

There was a bug when refactoring this with commits 5848a4d ("dhcp-server:
T1707: remove DHCP static-mappings from address pool") and 1182b44
("dhcp-server: T1707: bugfix on subsequent DHCP exclude addresses") that when
a static address assignemnt was using the last IP address from the specified
range.

This triggered the following error:

"DHCP range stop address x must be greater or equal to the range start
 address y!"