xdiff: refactor a function

xdiff: refactor a function

Use the standard "goto out" pattern rather than repeating very similar
code after checking for each error. This will simplify the next commit
that starts handling allocation failures that are currently ignored.
On error xdl_do_diff() frees the environment so we need to take care
to avoid a double free in that case. xdl_build_script() does not
assign a result unless it is successful so there is no possibility of
a double free if it fails.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>