When TLS is in use, checking the readiness of the underlying FD is insufficient
for determining if there is data available to be read. So before polling the
FD, check if there is any buffered data in the TLS layer and use that first.

ASTERISK-28562 #close
Reported by: Robert Sutton

Change-Id: I95fcb3e2004700d5cf8e5ee04943f0115b15e10d