Follow-up from "Resolve "dhcp pkt getIndex() and setIndex() should use int64_t instead of uint32_t type for ifindex""

The following discussion from !1363 (merged) should be addressed:

IfaceCollection::getIfaceInternal uses uint32_t instead of int32_t/int.

always check the return value of if_nametoindex which returns 0 on error.

ifa->ifa_index is uint32_t instead of int32_t/int.