Fix a map lookup that may throw exception. (#4098)

Summary:
`std::map::at(key)` throws std::out_of_range if key does not exist. Current
code does not handle this. Although this case is unlikely, I feel it's safe to
use `std::map::find`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4098

Differential Revision: D8753865

Pulled By: riversand963

fbshipit-source-id: 9a9ba43badb0fb5e0d24cd87903931fd12f3f8ec

db/external_sst_file_ingestion_job.cc

@@ -336,12 +336,14 @@ Status ExternalSstFileIngestionJob::GetIngestedFileInfo(
 
     // Set the global sequence number
     file_to_ingest->original_seqno = DecodeFixed64(seqno_iter->second.c_str());
-    file_to_ingest->global_seqno_offset = props->properties_offsets.at(
+    auto offsets_iter = props->properties_offsets.find(
         ExternalSstFilePropertyNames::kGlobalSeqno);
-
-    if (file_to_ingest->global_seqno_offset == 0) {
+    if (offsets_iter == props->properties_offsets.end() ||
+        offsets_iter->second == 0) {
+      file_to_ingest->global_seqno_offset = 0;
       return Status::Corruption("Was not able to find file global seqno field");
     }
+    file_to_ingest->global_seqno_offset = offsets_iter->second;
   } else if (file_to_ingest->version == 1) {
     // SST file V1 should not have global seqno field
     assert(seqno_iter == uprops.end());