diff: make sure we free all allocated resources

When the creation of one iterator fails, we need to free the prefix
and possibly one of the iterators. Make sure we do so.

src/diff.c

@@ -704,12 +704,17 @@ int git_diff_index_to_tree(
 	assert(repo && diff);
 
 	if (git_iterator_for_tree_range(&a, repo, old_tree, prefix, prefix) < 0 ||
-		git_iterator_for_index_range(&b, repo, prefix, prefix) < 0)
-		return -1;
+	    git_iterator_for_index_range(&b, repo, prefix, prefix) < 0)
+		goto on_error;
 
 	git__free(prefix);
 
 	return diff_from_iterators(repo, opts, a, b, diff);
+
+on_error:
+	git__free(prefix);
+	git_iterator_free(a);
+	return -1;
 }
 
 int git_diff_workdir_to_index(
@@ -723,12 +728,17 @@ int git_diff_workdir_to_index(
 	assert(repo && diff);
 
 	if (git_iterator_for_index_range(&a, repo, prefix, prefix) < 0 ||
-		git_iterator_for_workdir_range(&b, repo, prefix, prefix) < 0)
-		return -1;
+	    git_iterator_for_workdir_range(&b, repo, prefix, prefix) < 0)
+		goto on_error;
 
 	git__free(prefix);
 
 	return diff_from_iterators(repo, opts, a, b, diff);
+
+on_error:
+	git__free(prefix);
+	git_iterator_free(a);
+	return -1;
 }
 
 
@@ -744,12 +754,17 @@ int git_diff_workdir_to_tree(
 	assert(repo && old_tree && diff);
 
 	if (git_iterator_for_tree_range(&a, repo, old_tree, prefix, prefix) < 0 ||
-		git_iterator_for_workdir_range(&b, repo, prefix, prefix) < 0)
-		return -1;
+	    git_iterator_for_workdir_range(&b, repo, prefix, prefix) < 0)
+		goto on_error;
 
 	git__free(prefix);
 
 	return diff_from_iterators(repo, opts, a, b, diff);
+
+on_error:
+	git__free(prefix);
+	git_iterator_free(a);
+	return -1;
 }
 
 int git_diff_merge(