Reset pinnable slice before using it in Get() (#8154)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/6548.
If we do not reset the pinnable slice before calling get, we will see the following assertion failure
while running the test with multiple column families.
```
db_bench: ./include/rocksdb/slice.h:168: void rocksdb::PinnableSlice::PinSlice(const rocksdb::Slice&, rocksdb::Cleanable*): Assertion `!pinned_' failed.
```
This happens in `BlockBasedTable::Get()`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8154

Test Plan:
./db_bench --benchmarks=fillseq -num_column_families=3
./db_bench --benchmarks=readrandom -use_existing_db=1 -num_column_families=3

Reviewed By: ajkr

Differential Revision: D27587589

Pulled By: riversand963

fbshipit-source-id: 7379e7649ba40f046d6a4014c9ad629cb3f9a786

tools/db_bench_tool.cc

@@ -5287,11 +5287,11 @@ class Benchmark {
         ts_ptr = &ts_ret;
       }
       Status s;
+      pinnable_val.Reset();
       if (FLAGS_num_column_families > 1) {
         s = db_with_cfh->db->Get(options, db_with_cfh->GetCfh(key_rand), key,
                                  &pinnable_val, ts_ptr);
       } else {
-        pinnable_val.Reset();
         s = db_with_cfh->db->Get(options,
                                  db_with_cfh->db->DefaultColumnFamily(), key,
                                  &pinnable_val, ts_ptr);