res_rtp_asterisk: Fix dtls timer issues causing FRACKs and SEGVs

res_rtp_asterisk: Fix dtls timer issues causing FRACKs and SEGVs

In dtls_srtp_handle_timeout(), when DTLSv1_get_timeout() returned
success but with a timeout of 0, we were stopping the timer and
decrementing the refcount on instance but not resetting the
timeout_timer to -1.  When dtls_srtp_stop_timeout_timer()
was later called, it was atempting to stop a stale timer and could
decrement the refcount on instance again which would then cause
the instance destructor to run early.  This would result in either
a FRACK or a SEGV when ast_rtp_stop(0 was called.

According to the OpenSSL docs, we shouldn't have been stopping the
timer when DTLSv1_get_timeout() returned success and the new timeout
was 0 anyway.  We should have been calling DTLSv1_handle_timeout()
again immediately so we now reschedule the timer callback for
1ms (almost immediately).

Additionally, instead of scheduling the timer callback at a fixed
interval returned by the initial call to DTLSv1_get_timeout()
(usually 999 ms), we now reschedule the next callback based on
the last call to DTLSv1_get_timeout().

Resolves: #487