Resolve static check warnings in example code

Using cppcheck on libgit2 sources indicated two warnings in
example code.

merge.c was reported as having a memory leak. Fix applied
was to `free()` memory pointed to by `parents`.

init.c was reported as having a null pointer dereference
on variable arg. Function 'usage' was being called with
a null variable. Changed supplied parameter to empty string.

examples/init.c

@@ -244,5 +244,5 @@ static void parse_opts(struct opts *o, int argc, char *argv[])
 	}
 
 	if (!o->dir)
-		usage("must specify directory to init", NULL);
+		usage("must specify directory to init", "");
 }

examples/merge.c

@@ -220,6 +220,7 @@ static int create_merge_commit(git_repository *repo, git_index *index, merge_opt
 	check_lg2(git_repository_head(&head_ref, repo), "failed to get repo HEAD", NULL);
 	if (resolve_refish(&merge_commit, repo, opts->heads[0])) {
 		fprintf(stderr, "failed to resolve refish %s", opts->heads[0]);
+		free(parents);
 		return -1;
 	}