Add assert to array_slice::begin/end

Add assert to array_slice::begin/end

So while debugging PR 118320, I found it was useful to have
an assert inside array_slice::begin/end that the array slice isvalid
rather than getting an segfault. This adds an assert that is only
enabled for checking.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* vec.h (array_slice::begin): Assert that the
	slice is valid.
	(array_slice::end): Likewise.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>