mod_mam: Don't replace existing stanza ID

Preserve the original stanza ID for resent messages.  This avoids
storing duplicates in the MAM archive.

src/mod_mam.erl

@@ -404,6 +404,8 @@ get_stanza_id(#message{meta = #{stanza_id := ID}}) ->
     ID.
 
 -spec init_stanza_id(stanza(), binary()) -> stanza().
+init_stanza_id(#message{meta = #{stanza_id := _ID}} = Pkt, _LServer) ->
+    Pkt;
 init_stanza_id(Pkt, LServer) ->
     ID = p1_time_compat:system_time(micro_seconds),
     Pkt1 = strip_my_stanza_id(Pkt, LServer),