bridge_softmix: Fix some REMB bugs.

This change fixes a bug where a REMB collector may be
freed twice, and also tweaks REMB combining such that if
there is no bitrate from anyone (or there are no sources)
we report 0 instead of using an old bitrate.

ASTERISK-27804

Change-Id: Ia9dc9c150043890ee7ff85e9cdec007f1a77fcfd