Don't set the stack pointer to NULL on release builds

Author: brandtbucher

Include/internal/pycore_interpframe.h

@@ -171,7 +171,9 @@ _PyFrame_GetStackPointer(_PyInterpreterFrame *frame)
 {
     assert(frame->stackpointer != NULL);
     _PyStackRef *sp = frame->stackpointer;
+#ifndef NDEBUG
     frame->stackpointer = NULL;
+#endif
     return sp;
 }