libiptc: Avoid side-effect in memset() calls

These calls to memset() are passed a length argument which exceeds
t->target.u.user.name's length by one byte and hence overwrite
t->target.u.user.revision as well (relying upon no padding to happen
between both).

Avoid this obscure behaviour by passing the correct field size and
explicitly overwriting 'revision' field.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>