CheckConsistency fails after more then 1 files are ingested

We are running version 5.18.3. After we called 'IngestExternalFile' multiple times in a very short time, it occationally core dumped.
This is a case:
We ingest 4 files: 15450 15449 15445 15480 with global seq: 97985466 97985467 97985470 97985568
```
2019/04/28-12:50:28.948222 7fe6d6604700 [db/external_sst_file_ingestion_job.cc:250] [AddFile] External SST file 1163_000001.sst was ingested in L0 with path 015450.sst (global_seqno=97985466)
2019/04/28-12:50:28.958084 7fe678fe1700 [db/external_sst_file_ingestion_job.cc:250] [AddFile] External SST file 1155_000001.sst was ingested in L0 with path 015449.sst (global_seqno=97985467)
2019/04/28-12:50:30.440528 7fe6ece31700 [db/external_sst_file_ingestion_job.cc:250] [AddFile] External SST file 1208_000000.sst was ingested in L0 with path 015445.sst (global_seqno=97985470)
2019/04/28-12:50:33.563271 7fe6ece31700 [db/external_sst_file_ingestion_job.cc:250] [AddFile] External SST file 1208_000001.sst was ingested in L0 with path 015480.sst (global_seqno=97985568)
```
then the 4 files are compacted to file 15491:
```
2019/04/28-12:50:38.664788 7ff9577fe700 [db/compaction_job.cc:1688] [default] [JOB 185] Compacting 4@0 files to L0, score 10.26
2019/04/28-12:50:38.664802 7ff9577fe700 [db/compaction_job.cc:1692] [default] Compaction start summary: Base version 211 Base level 0, inputs: [15480(116MB) 15445(516MB) 15449(143MB) 15450(117MB)]
2019/04/28-12:50:49.359261 7ff9577fe700 [db/compaction_job.cc:1374] [default] [JOB 185] Generated table #15491: 893217 keys, 937243047 bytes
2019/04/28-12:50:49.370069 7ff9577fe700 [db/compaction_job.cc:1440] [default] [JOB 185] Compacted 4@0 files to L0 => 937243047 bytes
```
then memtable is flushed to file 15502:
```
2019/04/28-12:50:49.379557 7ff98d732700 [db/flush_job.cc:337] [default] [JOB 186] Level-0 flush table #15502: started
2019/04/28-12:50:49.382184 7ff98d732700 [db/flush_job.cc:377] [default] [JOB 186] Level-0 flush table #15502: 86869 bytes OK
```
then consistency check runs again the flushed file 15502 and failed:
db/version_builder.cc:
```
177 } else if (f1->fd.smallest_seqno <= f2->fd.smallest_seqno) {
178 fprintf(stderr,
179 "L0 files seqno %" PRIu64 " %" PRIu64 " vs. %" PRIu64
180 " %" PRIu64 "\n",
181 f1->fd.smallest_seqno, f1->fd.largest_seqno,
182 f2->fd.smallest_seqno, f2->fd.largest_seqno);
183 abort();
184 }
```
backtrace:
```
#2 0x0000000001ff2c99 in rocksdb::VersionBuilder::Rep::CheckConsistency (this=0x7ff958176dc0, vstorage=0x7ff9581c7de0)
at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_builder.cc:183
#3 0x0000000001ff3bb2 in rocksdb::VersionBuilder::Rep::SaveTo (this=0x7ff958176dc0, vstorage=0x7ff9581c7de0)
at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_builder.cc:364
#4 0x0000000001ff22ea in rocksdb::VersionBuilder::SaveTo (this=0x7ff9586e55d0, vstorage=0x7ff9581c7de0) at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_builder.cc:449
#5 0x0000000001dbf578 in rocksdb::VersionSet::ProcessManifestWrites (this=0x1c8c1f90, writers=std::deque with 1 elements = {...}, mu=0x1c8ae370, db_directory=0x42116b0,
new_descriptor_log=false, new_cf_options=0x0) at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_set.cc:2949
#6 0x0000000001dc1eab in rocksdb::VersionSet::LogAndApply (this=0x1c8c1f90, column_family_datas=..., mutable_cf_options_list=..., edit_lists=..., mu=0x1c8ae370,
db_directory=0x42116b0, new_descriptor_log=false, new_cf_options=0x0) at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_set.cc:3298
#7 0x0000000001fdf227 in rocksdb::VersionSet::LogAndApply (this=0x1c8c1f90, column_family_data=0x1c8c7890, mutable_cf_options=..., edit_list=..., mu=0x1c8ae370,
db_directory=0x42116b0, new_descriptor_log=false, column_family_options=0x0) at /home/likang/projects/likang/bytekv/third/rocksdb/db/version_set.h:788
```
```
(gdb) p *f1
$29 = {fd = {table_reader = 0x0, packed_number_and_path_id = 15502, file_size = 86869, smallest_seqno = 97985448, largest_seqno = 97985619}, smallest = {
rep_ = "\000\000\000\000\000\000\000\000@\016\001\000\000\005\000\001\000\001\000\000\000\000\000\000\000\001@$\327\005\000\000"}, largest = {
rep_ = "\351\003\000\000\000\000\000\000@user9582269196245260\251\217\331\000\037\212\231\025\001\061$\327\005\000\000"}, table_reader_handle = 0x0, stats = {
num_reads_sampled = {> = {_M_i = 0}, }}, compensated_file_size = 0, num_entries = 0, num_deletions = 0, raw_key_size = 0,
raw_value_size = 0, refs = 2, being_compacted = false, init_stats_from_file = false, marked_for_compaction = false}
(gdb) p *f2
$30 = {fd = {table_reader = 0x7fedbf351d20, packed_number_and_path_id = 15491, file_size = 937243047, smallest_seqno = 97985466, largest_seqno = 97985568}, smallest = {
rep_ = "\351\003\000\000\000\000\000\000@user459091711894869371\177\360\276\314\321M\231\025\001\276#\327\005\000\000"}, largest = {
rep_ = "\351\003\000\000\000\000\000\000@user8863435406276079852\273\005\036A\234E\231\025\001\273#\327\005\000\000"}, table_reader_handle = 0x7fea60323cd0, stats = {
num_reads_sampled = {> = {_M_i = 0}, }}, compensated_file_size = 937243047, num_entries = 893217, num_deletions = 0,
raw_key_size = 42771785, raw_value_size = 898576302, refs = 2, being_compacted = false, init_stats_from_file = true, marked_for_compaction = false}
```

### Expected behavior
CheckConsistency should pass

### Actual behavior
CheckConsistency failed

### Steps to reproduce the behavior
put some data to memtable, ingest at least 2 files whose key ranges are not overlapped with memtable, then compact the ingested files, then flush the memtable.