Check that the target address of a native call is not NULL

It's possible to pass MemoryAddress.NULL to CLinker::downcallHandle when linking, which will result in a segmentation fault when invoking the returned method handle.

Similarly, it is possible to invoke a segmentation fault by passing NULL to a virtual native method handle.

We can try to fail a bit faster here by checking that the target address is non-NULL before unboxing it.