fix(internal): Correct potential nil pointer err

fix(internal): Correct potential nil pointer err

With multiple user threads, if concurrently sending to the same
round robin group, there was a potential for a nil pointer
to be selected representing an endpoint.

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: I1ee57ff5d17998a5d097e583d497158f19eb300b