Fixing segmentation fault in db_bench

Summary:
Fixing segmentation fault when running db_bench

This seg fault happens because num_created is used without being initialized

Test Plan:
running db_bench using these arguments
bpl=10485760;overlap=10;mcz=2;del=300000000;levels=6;ctrig=4; delay=8; stop=12; wbn=3; mbc=20; mb=67108864;wbs=134217728; dds=0; sync=0; r=1000000; t=1; vs=800; bs=65536; cs=1048576; of=500000; si=1000000; ./db_bench --benchmarks=overwrite --disable_seek_compaction=1 --mmap_read=0 --statistics=1 --histogram=1 --num=$r --threads=$t --value_size=$vs --block_size=$bs --cache_size=$cs --bloom_bits=10 --cache_numshardbits=4 --open_files=$of --verify_checksum=1 --db=/home/tec/koko/ --sync=$sync --disable_wal=1 --compression_type=zlib --stats_interval=$si --compression_ratio=0.5 --disable_data_sync=$dds --write_buffer_size=$wbs --target_file_size_base=$mb --max_write_buffer_number=$wbn --max_background_compactions=$mbc --level0_file_num_compaction_trigger=$ctrig --level0_slowdown_writes_trigger=$delay --level0_stop_writes_trigger=$stop --num_levels=$levels --delete_obsolete_files_period_micros=$del --min_level_to_compress=$mcz --max_grandparent_overlap_factor=$overlap --stats_per_interval=1 --max_bytes_for_level_base=$bpl --use_existing_db=1

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D34881

db/db_bench.cc

@@ -820,6 +820,8 @@ struct DBWithColumnFamilies {
 
   DBWithColumnFamilies() : db(nullptr) {
     cfh.clear();
+    num_created = 0;
+    num_hot = 0;
   }
 
   DBWithColumnFamilies(const DBWithColumnFamilies& other)