Use list_to_existing_atom/1 wherever possible

src/ejabberd_cluster.erl

@@ -194,7 +194,7 @@ code_change(_OldVsn, State, _Extra) ->
 %%%===================================================================
 get_mod() ->
     Backend = ejabberd_option:cluster_backend(),
-    list_to_atom("ejabberd_cluster_" ++ atom_to_list(Backend)).
+    list_to_existing_atom("ejabberd_cluster_" ++ atom_to_list(Backend)).
 
 rpc_timeout() ->
     ejabberd_option:rpc_timeout().

src/ejabberd_oauth.erl

@@ -565,7 +565,7 @@ process(_Handlers, _Request) ->
 
 get_db_backend() ->
     DBType = ejabberd_option:oauth_db_type(),
-    list_to_atom("ejabberd_oauth_" ++ atom_to_list(DBType)).
+    list_to_existing_atom("ejabberd_oauth_" ++ atom_to_list(DBType)).
 
 
 %% Headers as per RFC 6749

src/ejabberd_router.erl

@@ -429,7 +429,7 @@ get_domain_balancing(From, To, LDomain) ->
 -spec get_backend() -> module().
 get_backend() ->
     DBType = ejabberd_option:router_db_type(),
-    list_to_atom("ejabberd_router_" ++ atom_to_list(DBType)).
+    list_to_existing_atom("ejabberd_router_" ++ atom_to_list(DBType)).
 
 -spec cache_nodes(module()) -> [node()].
 cache_nodes(Mod) ->

src/ejabberd_sm.erl

@@ -881,7 +881,7 @@ force_update_presence({LUser, LServer}) ->
 
 get_sm_backend(Host) ->
     DBType = ejabberd_option:sm_db_type(Host),
-    list_to_atom("ejabberd_sm_" ++ atom_to_list(DBType)).
+    list_to_existing_atom("ejabberd_sm_" ++ atom_to_list(DBType)).
 
 -spec get_sm_backends() -> [module()].
 

src/gen_mod.erl

@@ -353,7 +353,7 @@ db_mod(Opt, Host, Module) when is_binary(Host) orelse Host == global ->
 db_mod(Opt, Opts, Module) when is_map(Opts) ->
     db_mod(Opt, get_opt(Opt, Opts), Module);
 db_mod(_Opt, Type, Module) when is_atom(Type) ->
-    list_to_atom(atom_to_list(Module) ++ "_" ++ atom_to_list(Type)).
+    list_to_existing_atom(atom_to_list(Module) ++ "_" ++ atom_to_list(Type)).
 
 -spec loaded_modules(binary()) -> [atom()].
 loaded_modules(Host) ->