RTP_EncDec: avoid crashes caused by invalid RTP packets

The function f__RTP__dec accepts data (RTP packet) of arbitrary length, but it also accesses regions of the RTP header using fixed indexes. In case the RTP packet is shorter than those indexes, a segmentation failt occurs. This can be avoided by checking if the incoming RTP packet has at least a minimum size of 12 bytes. In this case we would return an empty ret_val.