Added assert() to make it clear to Coverity that both prev and next won't be null

src/osgPlugins/3ds/lib3ds/lib3ds_track.c

@@ -112,6 +112,8 @@ rot_key_setup(Lib3dsKey *prev, Lib3dsKey *cur, Lib3dsKey *next, float a[4], floa
     int i;
 
     assert(cur);
+    assert(prev || next);
+
     if (prev) {
         if (cur->value[3] > LIB3DS_TWOPI - LIB3DS_EPSILON) {
             lib3ds_quat_axis_angle(qm, cur->value, 0.0f);