Fix a bug in IterKey

Summary: IterKey set buffer_size_ to a wrong initial value, causing it to always allocate values from heap instead of stack if the key size is smaller. Fix it.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D18279

db/dbformat.h

@@ -280,7 +280,7 @@ class IterKey {
       delete[] key_;
     }
     key_ = space_;
-    buf_size_ = sizeof(buf_size_);
+    buf_size_ = sizeof(space_);
     key_size_ = 0;
   }