Make gimp_free_select_tool_halt() protected, and call it in
gimp_foreground_select_tool_set_trimap(), so that the free-select
subobject of the foreground-select tool is properly shut down
before switching to trimap mode.  In particular, this clears the
free-select tool widget at the right point; failing to do this
leads to CRITICALs later on.

(cherry picked from commit e1573323)