Add an assertion if a bad entity is given to a Sel node constructor
Add an assertion if a bad entity is given to a Sel node constructor

[r2587]

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