fix bug reported by new gcc warnings
fix bug reported by new gcc warnings

[r23962]

MatzeB/libfirm,davidgiven/libfirm,killbug2004/libfirm,killbug2004/libfirm,davidgiven/libfirm,killbug2004/libfirm,killbug2004/libfirm,MatzeB/libfirm,jonashaag/libfirm,MatzeB/libfirm,libfirm/libfirm,davidgiven/libfirm,8l/libfirm,libfirm/libfirm,8l/libfirm,8l/libfirm,8l/libfirm,libfirm/libfirm,davidgiven/libfirm,MatzeB/libfirm,libfirm/libfirm,davidgiven/libfirm,jonashaag/libfirm,jonashaag/libfirm,8l/libfirm,jonashaag/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,killbug2004/libfirm,MatzeB/libfirm,jonashaag/libfirm,killbug2004/libfirm,jonashaag/libfirm,MatzeB/libfirm,davidgiven/libfirm,killbug2004/libfirm,libfirm/libfirm,8l/libfirm,MatzeB/libfirm