Fix the bug of using freed memory introduced by recent plain table reader patch

Summary: Recent patch introduced a bug that if non-mmap mode is used, in prefix encoding case, there is a resizing of cur_key_ within the same prefix, we still read prefix from the released buffer. It fails ASAN tests and this commit fixes it.

Test Plan: Run the ASAN tests for the failing test case.

Reviewers: IslamAbdelRahman, yhchiang, anthony, igor, kradhakrishnan, rven

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D47457

table/plain_table_key_coding.cc

@@ -407,6 +407,7 @@ Status PlainTableKeyDecoder::NextPrefixEncodingKey(
           cur_key_.SetInternalKey(tmp, *parsed_key);
           parsed_key->user_key =
               Slice(cur_key_.GetKey().data(), prefix_len_ + size);
+          saved_user_key_ = cur_key_.GetKey();
         } else {
           cur_key_.Reserve(prefix_len_ + size);
           cur_key_.SetInternalKey(Slice(saved_user_key_.data(), prefix_len_),