GVariant has the concept of fixed-sized types (ie: types for which all
values of the type will have the same size).  Examples are booleans,
integers, doubles, etc.  Tuples containing only these types are also
fixed size.

When GVariant is trying to deal with a fixed-sized value for which it
doesn't have a sufficient backing store (eg: the case where a
fixed-sized value was created with g_variant_new_data() with an
incorrect number of bytes) it denotes this by setting the size of the
value to the correct fixed size but using a NULL data pointer.

This is well-documented in several code comments and also in the public
API documentation for g_variant_get_data() which describes the situation
number which NULL could be returned.

The decision to deal with this case in this way was changed at the last
minute around the time that GVariant was merged -- originally we had an
elaborate setup involving allocating an internal buffer of sufficient
size to be shared between...