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 <[email protected]>
Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

merge-ort.c

@@ -4683,14 +4683,14 @@ void merge_switch_to_result(struct merge_options *opt,
 void merge_finalize(struct merge_options *opt,
 		    struct merge_result *result)
 {
-	struct merge_options_internal *opti = result->priv;
-
 	if (opt->renormalize)
 		git_attr_set_direction(GIT_ATTR_CHECKIN);
 	assert(opt->priv == NULL);
 
-	clear_or_reinit_internal_opts(opti, 0);
-	FREE_AND_NULL(opti);
+	if (result->priv) {
+		clear_or_reinit_internal_opts(result->priv, 0);
+		FREE_AND_NULL(result->priv);
+	}
 }
 
 /*** Function Grouping: helper functions for merge_incore_*() ***/