Fix an uninitialized bool in exception print context.

`struct exception_print_context.need_close` was uninitialized.

Found by oss-fuzz in a test case running under the undefined behavior sanitizer.

https://oss-fuzz.com/testcase-detail/6217746058182656

```
Python/pythonrun.c:1241:28: runtime error: load of value 253, which is not a valid value for type 'bool'
    #0 0xbf2203 in print_chained cpython3/Python/pythonrun.c:1241:28
    #1 0xbea4bb in print_exception_cause_and_context cpython3/Python/pythonrun.c:1320:19
    #2 0xbea4bb in print_exception_recursive cpython3/Python/pythonrun.c:1470:13
    #3 0xbe9e39 in _PyErr_Display cpython3/Python/pythonrun.c:1517:9
```

Pretty obvious what the ommission was upon code inspection.

Author: gpshead

Python/pythonrun.c

@@ -1504,6 +1504,7 @@ _PyErr_Display(PyObject *file, PyObject *exception, PyObject *value, PyObject *t
     struct exception_print_context ctx;
     ctx.file = file;
     ctx.exception_group_depth = 0;
+    ctx.need_close = false;
     ctx.max_group_width = PyErr_MAX_GROUP_WIDTH;
     ctx.max_group_depth = PyErr_MAX_GROUP_DEPTH;