ubsan: Add another testcase for [0] array in the middle of struct [PR108894]

I think it is useful to cover also this, rather than just arrays at the
flexible array member positions.

2023-03-01  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/108894
	* c-c++-common/ubsan/bounds-16.c: New test.