netfilter: ebtables: abort if next_offset is too small

next_offset must be > 0, otherwise this loops forever.
The offset also contains the size of the ebt_entry structure
itself, so anything smaller is invalid.

Signed-off-by: Florian Westphal <fwestphal@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>