babl-fish: fix concurrent babl_fish()/process() calls with palette formats

babl-fish: fix concurrent babl_fish()/process() calls with palette formats

In babl_fish(), don't call _babl_fish_rig_dispatch() on existing
path fishes during each lookup.  We already call this function on
all path fishes upon creation (in particular, after commit
b34a9672c8c9a42923a8f0b2650af30b1efe635c), so this isn't necessary.

This fixes an issue where the _babl_fish_rig_dispatch() call in
babl_fish() would temporarily modify the value of babl->fish.data,
causing a concurrent babl_process[_rows]() call in another thread
to pass the wrong 'data' value down to the conversion function.  In
particular, this could cause palette conversions to segfault.