widl: Fix crash when trying to declare a fixed size array with a size_is attribute.

widl: Fix crash when trying to declare a fixed size array with a size_is attribute.

In this case type_array_get_conformance will return NULL. The code
here is actually replacing an existing array with an EXPR_VOID
conformance with the specified conformance, so issue an error if
type_array_get_conformance returns NULL or the conformance has already
been defined.