Fix a bug in lookaside related to birthmarks

   	21. Condition upd == NULL, taking true branch.

   	22. var_compare_op: Comparing upd to null implies that upd might be null.

   	23. Condition !skipped_birthmark, taking false branch.

219        if (upd == NULL && !skipped_birthmark) {

220                /*

221                 * Callers of this function set the cursor slot to an impossible

222                 * value to check we're not trying to return on-page values when

223                 * the update list should have been sufficient (which happens,

224                 * for example, if an update list was truncated, deleting some

225                 * standard update required by a previous modify update). Assert

226                 * the case.

227                 */

228                WT_ASSERT(session, cbt->slot != UINT32_MAX);

229

230                WT_ERR(__value_return(session, cbt));

CID 1383853 (#1 of 1): Dereference after null check (FORWARD_NULL)

24. var_deref_op: Dereferencing null pointer upd.

231        } else if (upd->type == WT_UPDATE_TOMBSTONE || skipped_birthmark)

232                WT_ERR(__wt_buf_set(session, &cursor->value, "", 0));

233        else

234                WT_ERR(__wt_buf_set(session,

235                    &cursor->value, upd->data, upd->size));