Forbid pointless http_add_cookie() with value==NULL and !replace
Forbid pointless http_add_cookie() with value==NULL and !replace

Coverity doesn't like it, because it would lead to dereferencing a NULL
pointer. It should never happen, since it's meaningless. So explicitly
return -EINVAL.

Signed-off-by: David Woodhouse <b460d66aaf00c296a3db1c1d9eeafc081d5f7d70@intel.com>

liiir1985/openconnect,brandt/openconnect,cernekee/openconnect,cernekee/openconnect,mveplus/openconnect-client,mtmiller/openconnect,cernekee/openconnect,mveplus/openconnect-client,brandt/openconnect,brandt/openconnect,liiir1985/openconnect,don-johnny/openconnect,lins05/openconnect,don-johnny/openconnect,cernekee/openconnect,nmav/openconnect-mine,liiir1985/openconnect,nmav/openconnect-mine,atyndall/openconnect,don-johnny/openconnect,mveplus/openconnect-client,atyndall/openconnect,matlockx/openconnect,atyndall/openconnect,brandt/openconnect,nmav/openconnect-mine,mtmiller/openconnect,matlockx/openconnect,mveplus/openconnect-client,lins05/openconnect,lins05/openconnect,nmav/openconnect-mine,matlockx/openconnect,mtmiller/openconnect,liiir1985/openconnect,atyndall/openconnect,don-johnny/openconnect,mtmiller/openconnect,matlockx/openconnect,lins05/openconnect