pack-objects: return early when computing write order fails
pack-objects: return early when computing write order fails

The function `compute_write_order` may return a `NULL`-pointer
when an error occurs. In such cases we jump to the `done`-label
where we try to clean up allocated memory. Unfortunately we try
to deallocate the `write_order` array, though, which may be NULL
here.

Fix this error by returning early instead of jumping to the
`done` label. There is no data to be cleaned up anyway.

joshtriplett/libgit2,leoyanggit/libgit2,stewid/libgit2,Corillian/libgit2,leoyanggit/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,joshtriplett/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,stewid/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,leoyanggit/libgit2,joshtriplett/libgit2,stewid/libgit2,saurabhsuniljain/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,stewid/libgit2,leoyanggit/libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2