Check that argv is set before operating on it.
Check that argv is set before operating on it.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@mit.edu>

ezyang/htmlpurifier,bartbutler/htmlpurifier,bartbutler/htmlpurifier,bfroehle/htmlpurifier,ezyang/htmlpurifier,Janson-Leung/htmlpurifier,stefanotorresi/htmlpurifier,Masterjoa/HTMLPurifier-standalone,r-kovalenko/htmlpurifier,Janson-Leung/htmlpurifier,Janson-Leung/htmlpurifier,stefanotorresi/htmlpurifier,ezyang/htmlpurifier,r-kovalenko/htmlpurifier,ezyang/htmlpurifier,bartbutler/htmlpurifier,bfroehle/htmlpurifier,Janson-Leung/htmlpurifier,r-kovalenko/htmlpurifier,Masterjoa/HTMLPurifier-standalone,stefanotorresi/htmlpurifier,r-kovalenko/htmlpurifier,bfroehle/htmlpurifier,stefanotorresi/htmlpurifier,bartbutler/htmlpurifier