roachpb: panic when comparing a Lease to a non-lease

Release note: None

pkg/roachpb/data.go

@@ -1876,7 +1876,7 @@ func (l *Lease) Equal(that interface{}) bool {
 		if ok {
 			that1 = &that2
 		} else {
-			return false
+			panic(fmt.Sprintf("attempting to compare lease to %T", that))
 		}
 	}
 	if that1 == nil {