config-parser: Fail on impossible send_broadcast/send_destination pair

config-parser: Fail on impossible send_broadcast/send_destination pair

If we add a rule like

    <allow send_destination="com.example" send_broadcast="true"/>

then it cannot possibly match anything, because to be a broadcast, the
message would have to have no destination. The only value of
send_destination that can be combined with send_broadcast="true" is
the wildcard "*", but by this point in the function we already
replaced "*" with NULL.

Adapted from an earlier implementation of send_broadcast by
Alban Crequy.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/92853
Reviewed-by: Philip Withnall <withnall@endlessm.com>