src: add fflush() to SnapshotData::ToFile()

Refs: #46491 (comment)
PR-URL: #46531
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

src/node_snapshotable.cc

@@ -866,6 +866,7 @@ void SnapshotData::ToFile(FILE* out) const {
   const std::vector<char> sink = ToBlob();
   size_t num_written = fwrite(sink.data(), sink.size(), 1, out);
   CHECK_EQ(num_written, 1);
+  CHECK_EQ(fflush(out), 0);
 }
 
 const SnapshotData* SnapshotData::FromEmbedderWrapper(