When preparing options for the two iterators that are about to be
diffed, we allocate a common prefix for both iterators depending on
the options passed by the user. We do not check whether the allocation
was successful, though. In fact, this isn't much of a problem, as using
a `NULL` prefix is perfectly fine. But in the end, we probably want to
detect that the system doesn't have any memory left, as we're unlikely
to be able to continue afterwards anyway.

While the issue is being fixed in the newly created function
`diff_prepare_iterator_opts`, it has been previously existing in the
previous macro `DIFF_FROM_ITERATORS` already.

When allocating tree iterator entries, we use GIT_ERROR_ALLOC_CHECK` to
check whether the allocation has failed. The macro will cause the
function to immediately return, though, leaving behind a partially
initialized iterator frame.

Fix the issue by manually checking for memory allocation errors and
using `goto done` in case of an error, popping the iterator frame.

When allocating new tree iterator frames, we zero out the allocated
memory twice. Remove one of the `memset` calls.