Fix warning about null pointer being passed to memcpy

I'd use std::copy_n(), but that's a host-only function
and the function being fixed is host/device, so I replace
memcpy() with a simple for loop.

VecGeom/base/Transformation3D.h

@@ -237,10 +237,10 @@ public:
   VECGEOM_FORCE_INLINE
   void Set(const Precision *trans, const Precision *rot, bool has_trans, bool has_rot)
   {
-    constexpr size_t kN3 = 3 * sizeof(Precision);
-    constexpr size_t kN9 = 9 * sizeof(Precision);
-    memcpy(fTranslation, trans, kN3);
-    memcpy(fRotation, rot, kN9);
+    for (auto &x : fTranslation)
+            x = *trans++;
+    for (auto &r : fRotation)
+            r = *rot++;
     fHasTranslation = has_trans;
     fHasRotation    = has_rot;
     fIdentity       = !fHasTranslation && !fHasRotation;