run_diff_files(): delay allocation of combine_diff_path

run_diff_files(): delay allocation of combine_diff_path

While looping over the index entries, when we see a higher level stage
the first thing we do is allocate a combine_diff_path struct for it. But
this can leak; if check_removed() returns an error, we'll continue to
the next iteration of the loop without cleaning up.

We can fix this by just delaying the allocation by a few lines.

I don't think this leak is triggered in the test suite, but it's pretty
easy to see by inspection. My ulterior motive here is that the delayed
allocation means we have all of the data needed to initialize "dpath" at
the time of malloc, making it easier to factor out a constructor
function.

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