Move the call to gimp_filter_tool_disable_color_picking() from the
filter-tool's dialog "unmap" handler to gimp_filter_tool_halt().
Since commit ec80a885, we
explicitly destroy the GUI when halting the filter tool, which
happens before the dialog's unmap handler is called, which could
potentially result in a dangling pointer to the active color-picker
widget in gimp_filter_tool_disable_color_picking().