read_var(): return NULL if file is empty
read_var(): return NULL if file is empty

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

xemul/mosaic,kolyshkin/mosaic,kolyshkin/mosaic,xemul/mosaic,kolyshkin/mosaic,xemul/mosaic