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
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Merged revisions 429868 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@429870 65c4cc65-6c06-0410-ace0-fbb531ad65f3