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/branches/11@376822 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -2593,7 +2593,7 @@ static void sip_websocket_callback(struct ast_websocket *session, struct ast_var
 		if (opcode == AST_WEBSOCKET_OPCODE_TEXT || opcode == AST_WEBSOCKET_OPCODE_BINARY) {
 			struct sip_request req = { 0, };
 
-			if (!(req.data = ast_str_create(payload_len))) {
+			if (!(req.data = ast_str_create(payload_len + 1))) {
 				goto end;
 			}