diff-lib: add idx/tree sanity check to oneway_diff

diff-lib: add idx/tree sanity check to oneway_diff

When looking just at the code in oneway_diff(), it seems possible for
both "idx" and "tree" to be NULL, in which case we'd potentially
segfault while checking the relative prefix.

But if you consider what these items actually mean, it shouldn't be
possible for both to be NULL. Let's add an assertion and a comment
documenting this. It might help human readers, but should also silence
static analyzers like Coverity which complain about the potential
segfault.

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