Add the KEY_END marker on key creation where missing

Add the KEY_END vararg where missing when creating a new key, either
using the C keyNew or the C++ kdb::Key, as done everywhere else.

This fixes crashes in some tests.
> Your explanation already gets to the point why it is really wrong! -> It has undefined behaviour. So if this was unclear: not your changes, but the initial code was wrong :-)

Ah ok, now I get what you said, sorry for the confusion.

> keyNew(0) does not have undefined behaviour [...]

Yes, I wrote that the changes in `src/bindings/cpp/tests/testcpp_key.cpp` are not needed; I'll try to update this pull request removing the changes to that file.