babl: re-creation of palette conversions must always set data.
If we allow conversions to be recreated for palettes, it will create a new identical conversion with collision number incremented in the name and no user data. Yet the user data is extremely important for palette conversions, because it contains the palette itself which is required by the palette conversion functions. Hence we end up crashing. Moreover, although I think that right now, only palette conversions use the data field, it might be used for other type of data in the future. In any case, we should always save this data along as it has been stored there for a reason. Finally in babl-palette code, make sure the user data is added to the models before creating the formats and conversions as we also ended up in crash-happy code when we didn't. See: gimp#6501
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