for_each_reflog_ent_reverse: turn leftover check into assertion

for_each_reflog_ent_reverse: turn leftover check into assertion

Our loop should always process all lines, even if we hit the
beginning of the file. We have a conditional after the loop
ends to double-check that there is nothing left and to
process it. But this should never happen, and is a sign of a
logic bug in the loop. Let's turn it into a BUG assertion.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>