Fail if tx blob is empty

Some node on the network apparently has a corrupted DB and is spewing
empty tx blobs onto the network.  Detect such a case rather than
broadcasting broken empty txes.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1998,6 +1998,9 @@ bool BlockchainLMDB::get_txpool_tx_blob(const crypto::hash& txid, std::string &b
   if (result != 0)
       throw1(DB_ERROR(lmdb_error("Error finding txpool tx blob: ", result).c_str()));
 
+  if (v.mv_size == 0)
+      throw1(DB_ERROR("Error finding txpool tx blob: tx is present, but data is empty"));
+
   bd.assign(reinterpret_cast<const char*>(v.mv_data), v.mv_size);
   return true;
 }