c++: Fix constexpr array ICE [PR98332]

c++: Fix constexpr array ICE [PR98332]

The element initializer was non-constant, so its CONSTRUCTOR element ended
up NULL, so unshare_constructor crashed trying to look at it.  This patch
fixes this in two places: First, by returning when we see a non-constant
initializer; second, by not crashing on NULL.

gcc/cp/ChangeLog:

	PR c++/98332
	* constexpr.c (unshare_constructor): Check for NULL.
	(cxx_eval_vec_init_1): Always exit early if non-constant.

gcc/testsuite/ChangeLog:

	PR c++/98332
	* g++.dg/cpp0x/constexpr-overflow3.C: New test.