luci-base: prevent tools.widgets.ZoneSelect tracebacks

luci-base: prevent tools.widgets.ZoneSelect tracebacks

follow-up fix for 0be4ad51a0bea3e1e3e6cd7d646e11ec6c722540

It seems to have been this way for a while, for zone selections
involving the 'any' ('*') choice. firewall forwards.js has no
'this.allowany;' property on the widgets, so the any choice is absent,
leading to a traceback, caused by the 'src' widget, even though
this code block operates on the 'dst' value. Certain combinations of
'src' and 'dst' also triggered tracebacks.

This might not be the correct behaviour for this widget, but it does
prevent tracebacks caused by null values.

Signed-off-by: Paul Donald <[email protected]>