res_rtp_asterisk: Correction for the limit which detects that a packet is DTLS.

First byte of DTLS packet shall be in range 20-63, not 20-64. Refer to RFC
https://tools.ietf.org/html/rfc5764#section-5.1.2 for correct values.

Change-Id: Iae6fa0d72b37c36a27fe40686e0ae6fba3afec31