Prevent admesh to reverse all facets twice in some mostly-random situ…

…ations.

Normalizing a null normal should still return a null normal in order to fix it properly later
instead of treating it as if it was a true normal and thus reversing the facet (and if that is
the first facet, all of the others would be reversed as well).

src/normals.c

@@ -355,7 +355,7 @@ void stl_normalize_vector(float v[])
   min_normal_length = 0.000000000001;
   if(length < min_normal_length)
     {
-      v[0] = 1.0;
+      v[0] = 0.0;
       v[1] = 0.0;
       v[2] = 0.0;
       return;