Fix missing header return in _populate_available_source_ids.

[axlan]

Fix crash when self.return_header is False.

Traceback (most recent call last):
  File "/home/jdiamond/src/nautilus/./point_one/platform/quectel-lg69t-evb/p1_runner/bin/extract_storage.py", line 215, in <module>
    main()
  File "/home/jdiamond/src/nautilus/./point_one/platform/quectel-lg69t-evb/p1_runner/bin/extract_storage.py", line 127, in main
    reader = MixedLogReader(input_path, message_types=(VersionInfoMessage,), return_header=False, return_bytes=True)
  File "/home/jdiamond/src/nautilus/venv_host_tools/lib/python3.10/site-packages/fusion_engine_client/parsers/mixed_log_reader.py", line 125, in __init__
    self._populate_available_source_ids()
  File "/home/jdiamond/src/nautilus/venv_host_tools/lib/python3.10/site-packages/fusion_engine_client/parsers/mixed_log_reader.py", line 643, in _populate_available_source_ids
    self.available_source_ids.add(header.source_identifier)
AttributeError: 'PlatformStorageDataMessage' object has no attribute 'source_identifier'

[adamshapiro0]

What's the issue here, is it a recursion thing or something? I'd definitely recommend a comment explaining this.

[axlan]

Nothing complicated, the lines in this function:

                    result = self.read_next()
                    header = result[0]

assume the header is returned.

[adamshapiro0]

Looks good, but please leave a note.