Fix crash in gdbpy_parse_register_id

I noticed that gdbpy_parse_register_id would assert if passed a Python
object of a type it was not expecting.  The included test case shows
this crash.  This patch fixes the problem and also changes
gdbpy_parse_register_id to be more "Python-like" -- it always ensures
the Python error is set when it fails, and the callers now simply
propagate the existing exception.

gdb/python/py-frame.c

@@ -253,10 +253,7 @@ frapy_read_register (PyObject *self, PyObject *args)
 
       if (!gdbpy_parse_register_id (get_frame_arch (frame), pyo_reg_id,
 				    &regnum))
-	{
-	  PyErr_SetString (PyExc_ValueError, "Bad register");
-	  return NULL;
-	}
+	return nullptr;
 
       gdb_assert (regnum >= 0);
       val = value_of_register (regnum, frame);