dtls_srtp: clear openssl error queue on errors

openssl has a nice global error queue, if an error occurs
we must read out the error and then call ERR_clear_error().
otherwise other users of openssl in the same process will
get our errors, and things will stop working.
account: add sample with outbound transport
rtp_stats: skip if no packets were sent/recv'd
audio: fix timestamp calculation for sending

according to RFC 3551 the RTP timestamp is independent of
number of channels and encoding. fix the timestamp calculation
for outgoing RTP packets, by dividing the total number of samples
on number of channels.

the bug was not affecting calls with mono audio, but was very
likely to affect calls with stereo audio (e.g. OPUS)