3027 installgrub can segfault when encountering bogus data on disk

@@ -50,10 +50,11 @@
         char                    *extra;
         multiboot_header_t      *mboot;
         uint32_t                mboot_off;
         uint32_t                buf_size;
         uint32_t                file_size;
+        uint32_t                extra_size;
 } ib_bootblock_t;
 
 typedef struct _ib_data {
         ib_device_t     device;
         ib_bootblock_t  bootblock;