3027 installgrub can segfault when encountering bogus data on disk

@@ -51,10 +51,11 @@
         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;