3027 installgrub can segfault when encountering bogus data on disk

*** 113,133 ****
          bblk_einfo_t            *einfo;
          uint32_t                cksum;
  
          assert(extra != NULL);
  
-         ext_header = (bb_header_ext_t *)extra;
-         cksum = compute_checksum(extra + sizeof (bb_header_ext_t),
-             ext_header->size);
-         BOOT_DEBUG("Extended information header checksum is %x\n", cksum);
- 
-         if (cksum != ext_header->checksum) {
-                 BOOT_DEBUG("Unable to find extended versioning information, "
-                     "data looks corrupted\n");
-                 return (NULL);
-         }
- 
          /*
           * Currently we only have one extra header so it must be encapsulating
           * the extended information structure.
           */
          einfo = (bblk_einfo_t *)(extra + sizeof (bb_header_ext_t));
--- 113,122 ----
*** 136,145 ****
--- 125,145 ----
                      "information, wrong magic identifier\n");
                  BOOT_DEBUG("Found %s, expected %s\n", einfo->magic,
                      EINFO_MAGIC);
                  return (NULL);
          }
+ 
+         ext_header = (bb_header_ext_t *)extra;
+         cksum = compute_checksum(extra + sizeof (bb_header_ext_t),
+             ext_header->size);
+         BOOT_DEBUG("Extended information header checksum is %x\n", cksum);
+ 
+         if (cksum != ext_header->checksum) {
+                 BOOT_DEBUG("Unable to find extended versioning information, "
+                     "data looks corrupted\n");
+                 return (NULL);
+         }
  
          return (einfo);
  }
  
  /*