Fix PR c++/38699

gcc/ChangeLog:

	PR c++/38699
	* c-common.c (fold_offsetof_1): Issue errors when the
	member designator of the offsetoff expression is not legitimate.

gcc/testsuite/ChangeLog:

	* c-c++-common/dfp/builtin-offsetof.c: New test.
	* g++.dg/other/offsetof6.C: Likewise.

From-SVN: r153843