If an initializer expr is to be NULL in a ctor initializer list, we
ICE in picflag_from_initializer and elsewhere.

If we're missing an initializer for a reference field, we report the
error, but then build a zero initializer to avoid the ICE.

for  gcc/cp/ChangeLog

	PR c++/84593
	* init.c (build_zero_init_1): Zero-initialize references.

for  gcc/testsuite/ChangeLog

	PR c++/84593
	* g++.dg/cpp1y/pr84593.C: New.

From-SVN: r258270