[Ada] Fix internal error on unchecked union with component clauses (2)

The issue arises when the unchecked union contains both a fixed part and
a variant part, and is subject to a full representation clause covering
all the components in all the variants, when the component clauses do not
align the variant boundaries with byte boundaries consistently.

gcc/ada/

	* gcc-interface/decl.cc (components_to_record): Use NULL recursively
	as P_GNU_REP_LIST for the innermost variant level in the unchecked
	union case with a fixed part.