Band-aid fix for SvtBroadcaster::Broadcast

...after 18d07b2affbbbfb2ff9152d3ad8d1744fe75ab47 "Rework SvtListener and
SvtBroadcaster internals" broke the case where SvtBroadcaster::Add/Remove is
called from within SvtBroadcaster::Broadcast, and at least CppunitTest_sc_ucalc
started to fail.

It should be evaluated whether the original SvtBroadcaster logic to handle this
was better than taking a copy here.

Change-Id: Ie47bccb8c5789e405349a9c8b89f30b1dba3cfba