res_http_websocket: Fix crash due to double freeing memory when receiving a payload length of zero.

Frames with a payload length of 0 were incorrectly handled in res_http_websocket.
Provided a frame with a payload had been received prior it was possible for a double
free to occur. The realloc operation would succeed (thus freeing the payload) but be
treated as an error. When the session was then torn down the payload would be
freed again causing a crash. The read function now takes this into account.

This change also fixes assumptions made by users of res_http_websocket. There is no
guarantee that a frame received from it will be NULL terminated.

ASTERISK-24472 #close
Reported by: Badalian Vyacheslav

Review: https://reviewboard.asterisk.org/r/4220/
Review: https://reviewboard.asterisk.org/r/4219/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@429270 f38db490-d61c-443f-a65b-d21fe96a405b