If we cast a user-supplied pointer to DBusBasicValue *, that's like
promising that the pointer is to an object at least as strictly aligned
as a DBusBasicValue. In practice, it often isn't: for example, when
we marshal a dbus_uint32_t, a pointer to it is only guaranteed to be
32-bit-aligned, whereas DBusBasicValue typically requires 64-bit
alignment.

Signed-off-by: Simon McVittie <smcv@collabora.com>