Possible optimization in conference bridge (thanks Michael Derfler for the patch)

[nanang]

The optimization proposed is in the conference bridge get_frame() function, brief desc:

• remove the redundant loop at the beginning (trivial) Note: basically this loop is iterating listeners, while the next loop is iterating transmitters, so they can't really be merged.
• the mixing step can be replaced with just copying the samples when there is only one transmitter for the listening port.

Original report can be found ​here.

[nanang]

In 4161:

Close #1532: optimization in conference bridge get_frame().

[nanang]

In 4162:

Re #1532: put back the 'redundant' loop at the beginning.