buffer: fix assert fail from JS API

Length arguments passed to SlowBuffer were coerced to Int32, not Uint32,
so passing a negative number would throw the following:

node: ../src/smalloc.cc:244: void node::smalloc::Alloc(): Assertion `length <= kMaxLength' failed.
Aborted (core dumped)

That has been fixed by coercing to Uint32 and comparing the value
against kMaxLength.