diff_delta_format_path: handle null filename

This fixes a crash in test cases
test_diff_parse__new_file_with_space_and_regenerate_patch
and
test_diff_parse__delete_file_with_space_and_regenerate_patch

Author: jorio

src/libgit2/diff_print.c

@@ -316,6 +316,12 @@ static int diff_print_oid_range(
 static int diff_delta_format_path(
 	git_str *out, const char *prefix, const char *filename)
 {
+	if (!filename)
+	{
+		/* don't prefix "/dev/null" */
+		return git_str_puts(out, "/dev/null");
+	}
+
 	if (git_str_joinpath(out, prefix, filename) < 0)
 		return -1;