GeomFromText in trigger has error
The error isn't coming from the trigger.   You could confirm this by dropping the trigger and you will see the same error.  

ERROR 1416 (22003): Cannot get geometry object from data you send to the GEOMETRY field


You didn't mention it in the question, but it looks like your user_geometry column is defined with a GEOMETRY data type.  To use a GEOMETRY data type, you have to insert a GEOMETRY object.  You can't coerce it later with a trigger because it's validated very early, before the BEFORE UPDATE trigger.

Remove the quotes and add a comma and you have a query that would work and not need the trigger:

INSERT INTO `users` (`id`, `name`, `email`, `password`, `user_geometry`) 
VALUES (NULL, 'arash', 'arash@host.com', 'sssssss', POINT(-0.23,169.93));


The POINT() function takes 2 arguments and returns a geometry object.