Refactor: Animation, prevent incrementing null pointer

Only do `next++` if `next` is a non-`NULL` pointer. This doesn't change
the functionality of the code, but it does prevent a sanitizer warning.

No functional changes.

source/blender/blenkernel/intern/curve.cc

@@ -3386,7 +3386,7 @@ static void calchandlesNurb_intern(Nurb *nu, eBezTriple_Flag handle_sel_flag, bo
         next = nullptr;
       }
     }
-    else {
+    else if (next != nullptr) {
       next++;
     }