Add check for storageclass in interface.c

[Russ Allbery]

Sebastian Wiesinger <inn at tracker.fire-world.de> writes:

> storage/interface.c has no test to prevent a storage class >255, but the
> rest of the code assumes that the storage class is below 255 (is an
> integer).  There could be problems if someone uses a storage class >255.

Thanks, added a check.

-- 
Russ Allbery (rra at stanford.edu)             <http://www.eyrie.org/~eagle/>

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