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>