Fix broken pointer overflow check ns_name_unpack()

Many compilers may optimize away the overflow check `msg + l < msg',
where `msg' is a pointer and `l' is an integer, because pointer
overflow is undefined behavior in C.

Use a safe precondition test `l >= eom - msg' instead.
Bug: 13219633

Change-Id: I3fca2125834073cc36d7e9c4e586e97842265a59