db_lmdb: set same packing format for output_data_t and pre_rct_output_data_t

For safety, though it seems to have been the case already.

Also add a comment about the necessary layout identity.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -51,12 +51,15 @@ using epee::string_tools::pod_to_hex;
 namespace
 {
 
+#pragma pack(push, 1)
+// This MUST be identical to output_data_t, without the extra rct data at the end
 struct pre_rct_output_data_t
 {
   crypto::public_key pubkey;       //!< the output's public key (for spend verification)
   uint64_t           unlock_time;  //!< the output's unlock time (or height)
   uint64_t           height;       //!< the height of the block which created the output
 };
+#pragma pack(pop)
 
 template <typename T>
 inline void throw0(const T &e)