CDC-NCM: avoid overflow in sanity checking

In the Linux kernel, the following vulnerability has been resolved: CDC-NCM: avoid overflow in sanity checking A broken device may give an extreme offset like 0xFFF0 and a reasonable length for a fragment. In the sanity check as formulated now, this will create an integer overflow, defeating the sanity check. Both offset and offset + len need to be checked in such a manner that no overflow can occur. And those quantities should be unsigned.