Fix passwd_hash return type error

src/emqttd_auth_mod.erl

@@ -63,12 +63,12 @@ passwd_hash(sha256, Password)  ->
 passwd_hash(pbkdf2, {Salt, Password, Macfun, Iterations, Dklen}) ->
     case pbkdf2:pbkdf2(Macfun, Password, Salt, Iterations, Dklen) of
         {ok, Hexstring} -> pbkdf2:to_hex(Hexstring);
-        {error, Error} -> lager:error("PasswdHash with pbkdf2 error:~p", [Error]), error
+        {error, Error} -> lager:error("PasswdHash with pbkdf2 error:~p", [Error]), <<>>
     end;
 passwd_hash(bcrypt, {Salt, Password}) ->
     case bcrypt:hashpw(Password, Salt) of
         {ok, HashPassword} -> list_to_binary(HashPassword);
-        {error, Error}-> lager:error("PasswdHash with bcrypt error:~p", [Error]), error
+        {error, Error}-> lager:error("PasswdHash with bcrypt error:~p", [Error]), <<>>
     end.
 
 hexstring(<<X:128/big-unsigned-integer>>) ->