Ethernet: Add a length check

Ethernet: Add a length check

This fix some undefined behaviors at runtime.

The errors were like:

print-ether.c:241:11: runtime error: unsigned integer overflow:
  1 - 2 cannot be represented in type 'unsigned int'
print-ether.c:242:11: runtime error: unsigned integer overflow:
  1 - 2 cannot be represented in type 'unsigned int'

Moreover:
Fix indentation.