Add a check for NULL nlr_top in nlr_jump.

If no nlr_buf has been pushed, and an nlr_jump is called, then control
is transferred to nlr_jump_fail (which should bail out with a fatal
error).

py/nlr.h

@@ -27,6 +27,11 @@ unsigned int nlr_push(nlr_buf_t *);
 void nlr_pop(void);
 void nlr_jump(void *val) __attribute__((noreturn));
 
+// This must be implemented by a port.  It's called by nlr_jump
+// if no nlr buf has been pushed.  It must not return, but rather
+// should bail out with a fatal error.
+void nlr_jump_fail(void *val);
+
 // use nlr_raise instead of nlr_jump so that debugging is easier
 #ifndef DEBUG
 #define nlr_raise(val) nlr_jump(val)