c++: ICE with pack indexing empty pack [PR117898]

c++: ICE with pack indexing empty pack [PR117898]

Here we ICE with a partially-substituted pack indexing.  The pack
expanded to an empty pack, which we can't index.  It seems reasonable
to detect this case in tsubst_pack_index, even before we substitute
the index.  Other erroneous cases can wait until pack_index_element
where we have the index.

	PR c++/117898

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_index): Detect indexing an empty pack.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing2.C: Adjust.
	* g++.dg/cpp26/pack-indexing12.C: New test.