Fix uninitialized stack memory and NULL ptr dereference in stash_to_index

Caught by static analysis.

src/stash.c

@@ -173,7 +173,7 @@ static int stash_to_index(
 	git_index *index,
 	const char *path)
 {
-	git_index *repo_index;
+	git_index *repo_index = NULL;
 	git_index_entry entry = {{0}};
 	struct stat st;
 	int error;
@@ -187,7 +187,7 @@ static int stash_to_index(
 		return error;
 
 	git_index_entry__init_from_stat(&entry, &st,
-		(repo_index != NULL || !repo_index->distrust_filemode));
+		(repo_index == NULL || !repo_index->distrust_filemode));
 
 	entry.path = path;