submodule: add better error handling to is_path_occupied

Author: Carson Howard

src/submodule.c

@@ -176,7 +176,10 @@ static int is_path_occupied(bool *occupied, git_repository *repo, const char *pa
 	if ((error = git_path_to_dir(&dir)) < 0)
 		goto out;
 
-	if ((error = git_index_find_prefix(NULL, index, dir.ptr)) == 0) {
+	if ((error = git_index_find_prefix(NULL, index, dir.ptr)) < 0 && error != GIT_ENOTFOUND)
+    goto out;
+
+	if (!error) {
 		giterr_set(GITERR_SUBMODULE,
 			"Directory '%s' already exists in the index", path);
 		*occupied = true;