c++: variable tmpl partial specialization [PR108468]

Generally we expect TPARMS_PRIMARY_TEMPLATE to be set, but sometimes it
isn't for partial instantiations.  This ought to be improved, but it's
trivial to work around it in this case.

	PR c++/108468

gcc/cp/ChangeLog:

	* pt.cc (unify_pack_expansion): Check that TPARMS_PRIMARY_TEMPLATE
	is non-null.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ78.C: New test.