Each of the following cleans up tag refcounting, fixes access to
released memory, or other small glitches. Not sure which change
actually fixed the bug:

gimp_data_remove_tag(): remove the found tag, not the passed in tag
(which is to be treated only as a value for comparison).

gimp_tagged_remove_tag(): don't continue the loop after the tag to
remove has been found, there can only be one matching tag, and the
list element has become invalid.

gimptagentry.c: keep references around for the members of
entry->common_tags, and make sure the references are always dropped
properly. In assign_tags(), reference the "add" and "remove" lists for
paranoia and safety reasons.