Fixed didIO not being set with no block_cache

Summary:
In `Table::BlockReader()` when there was no block cache `didIO` was not set.

This didn't seem to matter as `didIO` is only used to trigger seek compactions. However, I would like it if someone else could check that is the case.

Test Plan: `make check OPT="-g -O3"`

Reviewers: dhruba, vamsi

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8133

table/table.cc

@@ -207,6 +207,9 @@ Iterator* Table::BlockReader(void* arg,
       if (s.ok()) {
         block = new Block(contents);
       }
+      if (didIO != NULL) {
+        *didIO = true; // we did some io from storage
+      }
     }
   }