When _PyOptimizer_BackEdge fails, restore next_instr

This fixes a nasty bug in exception handling.
Apparently some part of the computation that determines
where the exception handler is uses next_instr,
which was pointing at the JUMP_BACKWARD instruction instead of
where the error was occurring.

Python/bytecodes.c

@@ -2234,6 +2234,7 @@ dummy_func(
                 frame = _PyOptimizer_BackEdge(frame, here, next_instr, stack_pointer);
                 if (frame == NULL) {
                     frame = cframe.current_frame;
+                    next_instr = frame->prev_instr + 1;
                     goto error;
                 }
                 assert(frame == cframe.current_frame);