Fix warning about uninitialized variable inside stream.peak():

Fix warning about uninitialized variable inside stream.peak():

openmw/mwgui/bookpage.cpp:394:13: warning: ‘*((void*)& stream +24)’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>