merge-ort: fix calling merge_finalize() with no intermediate merge

merge-ort: fix calling merge_finalize() with no intermediate merge

If some code sets up the data structures for a merge, but then never
actually performs one before calling merge_finalize(), then
merge_finalize() wouldn't notice that result->priv was NULL and
return early, resulting in following that NULL pointer and getting
a segfault.  There is currently no code in the git codebase that does
this, but this issue was found during testing of some proposed patches
that had the following structure:

    struct merge_options merge_opt;
    struct merge_result result;

    init_merge_options(&merge_opt, the_repository);
    memset(&result, 0, sizeof(result));

    <do N merges, for some value of N>

    merge_finalize(&merge_opt, &result);

where some flags could cause the code to have N=0, i.e. doing no merges.
Add a check for result->priv being NULL and return early to avoid a
segfault in these kinds of cases.

While at it, ensure the FREE_AND_NULL() in the function does something
useful with the nulling aspect, namely sets result->priv to NULL rather
than a mere temporary.

Reported-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>