Fix potential memory leak of scratch buffer (#6879)

Summary:
If `req.scratch` is an internally allocated buffer, but `raw_block_contents` is not constructed to own `req.scratch`, then `req.scratch` will be leaked.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6879

Test Plan: make asan_check

Reviewed By: anand1976

Differential Revision: D21728498

Pulled By: cheng-chang

fbshipit-source-id: 8fc6a4f2543918c565ddc16ecfad1807eb9a42cf

table/block_based/block_based_table_reader.cc

@@ -1717,7 +1717,8 @@ void BlockBasedTable::RetrieveMultipleBlocks(
     FSReadRequest& req = read_reqs[req_idx];
     Status s = req.status;
     if (s.ok()) {
-      if (req.result.size() != req.len) {
+      if ((req.result.size() != req.len) ||
+          (req_offset + block_size(handle) > req.result.size())) {
         s = Status::Corruption(
             "truncated block read from " + rep_->file->file_name() +
             " offset " + ToString(handle.offset()) + ", expected " +
@@ -1726,15 +1727,6 @@ void BlockBasedTable::RetrieveMultipleBlocks(
     }
 
     BlockContents raw_block_contents;
-    size_t cur_read_end = req_offset + block_size(handle);
-    if (cur_read_end > req.result.size()) {
-      s = Status::Corruption(
-          "truncated block read from " + rep_->file->file_name() + " offset " +
-          ToString(handle.offset()) + ", expected " + ToString(req.len) +
-          " bytes, got " + ToString(req.result.size()));
-    }
-
-    if (s.ok()) {
     if (!use_shared_buffer) {
       // We allocated a buffer for this block. Give ownership of it to
       // BlockContents so it can free the memory
@@ -1750,11 +1742,11 @@ void BlockBasedTable::RetrieveMultipleBlocks(
       raw_block_contents =
           BlockContents(Slice(req.result.data() + req_offset, handle.size()));
     }
-
 #ifndef NDEBUG
       raw_block_contents.is_raw_block = true;
 #endif
-      if (options.verify_checksums) {
+
+    if (s.ok() && options.verify_checksums) {
       PERF_TIMER_GUARD(block_checksum_time);
       const char* data = req.result.data();
       uint32_t expected =
@@ -1769,7 +1761,6 @@ void BlockBasedTable::RetrieveMultipleBlocks(
                                             handle.size() + 1, expected);
       TEST_SYNC_POINT_CALLBACK("RetrieveMultipleBlocks:VerifyChecksum", &s);
     }
-    }
 
     if (s.ok()) {
       // When the blocks share the same underlying buffer (scratch or direct io