derhass reports that Raspbian g++-4.8.2 rejects vecmat.cpp for
-Wmaybe-uninitialized on t0, t1. Fix it by adding code to return 0 if
vm_vec_copy_normalize leaves the temporary uninitialized.
Reported-by: derhass <https://github.com/dxx-rebirth/dxx-rebirth/issues/55>