ipam: Do not deref nil in GET /ipinfo/tracker if tracker doesn't exist
ipam: Do not deref nil in GET /ipinfo/tracker if tracker doesn't exist

In some cases, we do not create any IPAM tracker, which leads
to a panic when GET'ing /ipinfo/tracker.

weaveworks/weave,weaveworks/weave,weaveworks/weave,weaveworks/weave