Fix chan_sip websocket payload handling

Websocket by default doesn't return an ast_str for the payload received. When 
converting it to an ast_str on chan_sip the last character was being omitted, 
because ast_str functions expects that the given length includes the trailing 
0x00. payload_len only has the actual string length without counting the 
trailing zero.

For most cases this passed unnoticed as most of SIP messages ends with \r\n.

(closes issue ASTERISK-20745)
Reported by: Iñaki Baz Castillo
Review: https://reviewboard.asterisk.org/r/2219/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376820 65c4cc65-6c06-0410-ace0-fbb531ad65f3