[core] Stop the timer error log on RTP sessions without a timer (#3128)

`read_rtp_packet()` runs `switch_core_timer_check()` on every empty
read, and that logs an error when `rtp_session->timer` is zeroed, the
normal state for T.38, proxy media, and a disabled timer. Test
`timer.timer_interface` first so those sessions stay quiet; the branch
itself is not reachable without a timer, so packet handling is unchanged.

src/switch_rtp.c

@@ -6556,6 +6556,7 @@ static switch_status_t read_rtp_packet(switch_rtp_t *rtp_session, switch_size_t
 		}
 	} else if (!switch_cmp_addr(rtp_session->from_addr, rtp_session->remote_addr, SWITCH_FALSE) &&
 				!switch_rtp_test_flag(rtp_session, SWITCH_RTP_FLAG_AUTOADJ) &&
+				rtp_session->timer.timer_interface &&
 				switch_core_timer_check(&rtp_session->timer, SWITCH_FALSE) == SWITCH_STATUS_FALSE) {
 			/**
 			* nothing has been read from socket. We are not in auto-adjustment window anymore