`Parser3::parse` previously formed `buf + len` immediately, so passing a
negative `len` from the C API caused undefined pointer arithmetic. Add an
explicit `len < 0` check that returns `WeaselJson_REJECT` (and makes the
rejected state sticky) before any `buf + len` computation.
Also document the non-negative length precondition in the public header
and add a regression test.
Closes#24
weaselbot