bpo-45256: gdb: fix printing AttributeError in py-locals when no frame is available

```
Unable to read information on python frame
Python Exception <class 'AttributeError'>: 'NoneType' object has no attribute 'co_name'
```

Regression in commit b4903af. While
refactoring the code into a while loop, the previous early return when
no frame exists went missing. We have just printed a message that we
cannot get information about this, so the frame will be None, and we
cannot attempt to use it.

Discovered on python 3.11, in python 3.12a2 this should error out with
`.is_shim()` instead of `co_name`.

Author: eli-schwartz

Misc/NEWS.d/next/Tools-Demos/2022-12-29-19-22-11.bpo-45256.a0ee_H.rst

@@ -0,0 +1 @@
+Fix confusing AttributeError in python-gdb.py when py-locals is used without a frame.

Tools/gdb/libpython.py

@@ -2108,6 +2108,7 @@ def invoke(self, args, from_tty):
         while True:
             if not pyop_frame:
                 print(UNABLE_READ_INFO_PYTHON_FRAME)
+                break
             if pyop_frame.is_shim():
                 break