c++: constexpr and array[0] [PR101029]

build_vec_init_elt exits early if we're initializing a zero-element array,
so build_vec_init needs to do the same to avoid trying to instantiate things
after we've already started throwing important bits away.

	PR c++/101029

gcc/cp/ChangeLog:

	* init.c (build_vec_init): Shortcut [0] case.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/array4.C: New test.