Commit 47a6f744d split out redundant code, but accidentally stored
temporaries in a `signed short` instead of a `fix` as they should have
been. This truncated some values, causing odd results whenever
quaternions were used. Fix the problem by storing the intermediate
results in a `fix`.
Fixes: 47a6f744d8 ("Factor out vms_quaternion_from_matrix division")
Reported-by: Ninjared <https://forum.dxx-rebirth.com/showthread.php?tid=1113>