In gimp_filter_tool_halt(), explicitly clear the GUI container
before clearing filter_tool->config, since the tool might be halted
during the GUI dialog's delete event, in which case the GUI will
only be implicitly destroyed *after* the function returns.  The
destruction of the GUI might fire signals whose handlers rely on
filter_tool->config, so we need to make sure it happens while it's
still alive.

In particular, this fixes a CRITICAL in the threshold tool, which
occurs due to the histogram view's "range-changhed" signal being
fired during its destruction, and its handler accessing
filter_tool->config.