mod_mam: Don't strip offline message stanza IDs

As mod_offline currently doesn't preserve metadata, add an explicit
check for messages retrieved from offline storage to avoid stripping
their stanza IDs.

Thanks to Zuglufttier for spotting this.

src/mod_mam.erl

@@ -419,6 +419,8 @@ get_stanza_id(#message{meta = #{stanza_id := ID}}) ->
 -spec init_stanza_id(stanza(), binary()) -> stanza().
 init_stanza_id(#message{meta = #{stanza_id := _ID}} = Pkt, _LServer) ->
     Pkt;
+init_stanza_id(#message{meta = #{from_offline := true}} = Pkt, _LServer) ->
+    Pkt;
 init_stanza_id(Pkt, LServer) ->
     ID = p1_time_compat:system_time(micro_seconds),
     Pkt1 = strip_my_stanza_id(Pkt, LServer),