We were aborting when build_base_path returned an error because of the
derived class not being complete yet, which wasn't considered by the assert.
Fixed by checking for complete type first.  The semantics.c change avoids
a duplicate error message.

	* semantics.c (finish_offsetof): Handle error_mark_node.
	* typeck.c (build_class_member_access_expr): Call
	complete_type_or_maybe_complain before converting to base.

From-SVN: r270135