res_pjsip_sdp_rtp: Restore removed NULL check.

When sending an RTP keepalive, we need to be sure we're not dealing with
a NULL RTP instance. There had been a NULL check, but the commit that
added the rtp_timeout and rtp_hold_timeout options removed the NULL
check.

Change-Id: I2d7dcd5022697cfc6bf3d9e19245419078e79b64