MappingSystem#getWidestNegativePrefix(Eid) returns incorrect results

One of the uses of the MappingSystem#getWidestNegativePrefix(Eid) method is to generate the negative reply to southbound originated Map-Requests which don't get an actual stored mapping as a result. Right now, this method will try to get the widest negative on the NB and if that fails (null is returned) then on the SB.

NB negative prefix will only fail if we try to use an EID that has match, in which case we don'tget to call the method in the first place for this use case, so we never actually get to looking up the widest negative for SB.

Suppose we have a mapping for 192.0.2.0/24 in NB and 10.0.0.0/32 in SB. A mapping lookup for 11.1.1.1 will return 0.0.0.0/1 as the negative mapping, covering 10.0.0.0/8. Instead, it should return the widest negative between 10.0.0.0/32 and 192.0.2.0/24.

So the correct behavior should be to look up both NB and SB widest negative prefixes and return their intersection.