3027 installgrub can segfault when encountering bogus data on disk

*** 51,60 ****
--- 51,61 ----
          char                    *file;
          char                    *extra;
          multiboot_header_t      *mboot;
          uint32_t                mboot_off;
          uint32_t                file_size;
+         uint32_t                extra_size;
          uint32_t                buf_size;
          uint32_t                first_sector;
          uint32_t                pcfs_first_sectors[2];
  } ig_stage2_t;