decl.c (gnat_to_gnu_entity): Make sure the size of a padding type built around the return type is positive.

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Function>: Make sure
	the size of a padding type built around the return type is positive.
	* gcc-interface/trans.c (gnat_to_gnu) <N_Simple_Return_Statement>:
	Use INIT_EXPR instead of MODIFY_EXPR to assign to the return object.

From-SVN: r224697