res_http_websocket.c: Fix incorrect use of sizeof in ast_websocket_write().

This won't fix the reported issue but it is an incorrect use of sizeof.

ASTERISK-24566
Reported by:  Badalian Vyacheslav
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Merged revisions 429867 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@429868 f38db490-d61c-443f-a65b-d21fe96a405b