Fixed undefined variable in logging

[radarhere]

Resolves #5555

In TiffImagePlugin, values might not be defined when it is comes to be used in logging.

if is_ifd:
    ...
else:
    values = value if isinstance(value, tuple) else (value,)
    ...

...
msg += " - value: " + (
    "<table: %d bytes>" % len(data) if len(data) >= 16 else str(values)
)

This PR fixes it by ensuring that values is always defined.