libstdc++: Add assertions to std::list::pop_{front,back}

libstdc++: Add assertions to std::list::pop_{front,back}

The recently-approved Standard Library Hardening proposal (P3471R4)
gives pop_front and pop_back member functions hardened preconditions,
but std::list was missing assertions on them. Our other sequence
containers do have assertions on those members.

libstdc++-v3/ChangeLog:

	* include/bits/stl_list.h (list::pop_front, list::pop_back):
	Add non-empty assertions.

Reviewed-by: Patrick Palka <ppalka@redhat.com>