libnet/ipams/default: improve address pools validation

Nothing was validating whether address pools' `base` prefix
were larger than the target subnet `size` they're associated to. As
such invalid address pools would yield no subnet, the error could go
unnoticed.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

libnetwork/ipams/defaultipam/allocator.go

@@ -95,6 +95,9 @@ func splitByIPFamily(s []*ipamutils.NetworkToSplit) ([]*ipamutils.NetworkToSplit
 		if !n.Base.IsValid() || n.Size == 0 {
 			return []*ipamutils.NetworkToSplit{}, []*ipamutils.NetworkToSplit{}, fmt.Errorf("network at index %d (%v) is not in canonical form", i, n)
 		}
+		if n.Base.Bits() > n.Size {
+			return []*ipamutils.NetworkToSplit{}, []*ipamutils.NetworkToSplit{}, fmt.Errorf("network at index %d (%v) has a smaller prefix (/%d) than the target size of that pool (/%d)", i, n, n.Base.Bits(), n.Size)
+		}
 
 		n.Base, _ = n.Base.Addr().Unmap().Prefix(n.Base.Bits())