Simplify fixnum division slightly

* src/data.c (arith_driver): Streamline fixnum division a bit
more, and add a comment about why overflow is impossible.
This responds to a private comment by Stefan Monnier.
1 job for master in 59 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #4132


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1