Thanks Vitaly, but instead of inverting the IF-condition I would prefer to make my function more robust, since it is kind of security-related and I might forget about the special IF-condition later when using it elsewhere...
Merlin's point about inlining and SQL language functions not withstanding:
You should consider writing a variation of the check_user function that returns VOID or raises an exception and use is like an assertion.
I would consider raising an exception if in_auth is NULL as I'd potentially consider such a situation to represent mis-usage of the function which should gets it own error instead of simply indicating that the validation failed.
