stun: Fix a use of a function with an aggregate return value

stun: Fix a use of a function with an aggregate return value

div() has an aggregate return, which GCC doesn’t like, although this
seems like a pretty pointless warning because div_t is the same size as
a pointer on 64-bit platforms (probably) and hardly going to cause
performance problems by passing it by value.

Anyway, it seems easier to simplify the code by using explicit / and %
operators inline, than it does to add pragmas and shut the warning up.