Fix async_io regression in scans (#10939)

Fix async_io regression in scans (#10939)

Summary:
Fix async_io regression in scans due to incorrect check which was causing the valid data in buffer to be cleared during seek.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10939

Test Plan:
- stress tests  export CRASH_TEST_EXT_ARGS="--async_io=1"
    make crash_test -j32
- Ran db_bench command which was caught the regression:
./db_bench --db=/rocksdb_async_io_testing/prefix_scan --disable_wal=1 --use_existing_db=true --benchmarks="seekrandom" -key_size=32 -value_size=512 -num=50000000 -use_direct_reads=false -seek_nexts=963 -duration=30 -ops_between_duration_checks=1 --async_io=true --compaction_readahead_size=4194304 --log_readahead_size=0 --blob_compaction_readahead_size=0 --initial_auto_readahead_size=65536 --num_file_reads_for_auto_readahead=0 --max_auto_readahead_size=524288

seekrandom   :    3777.415 micros/op 264 ops/sec 30.000 seconds 7942 operations;  132.3 MB/s (7942 of 7942 found)

Reviewed By: anand1976

Differential Revision: D41173899

Pulled By: akankshamahajan15

fbshipit-source-id: 2d75b06457d65b1851c92382565d9c3fac329dfe