use "NULL" instead of "0" in execl params (4 places) 	patch from: Andreas Gruenbacher <agruen@suse.de>
use "NULL" instead of "0" in execl params (4 places)
	patch from: Andreas Gruenbacher <agruen@suse.de>

ellson/graphviz,pixelglow/graphviz,tkelman/graphviz,jho1965us/graphviz,jho1965us/graphviz,pixelglow/graphviz,jho1965us/graphviz,tkelman/graphviz,jho1965us/graphviz,tkelman/graphviz,pixelglow/graphviz,BMJHayward/graphviz,ellson/graphviz,tkelman/graphviz,kbrock/graphviz,BMJHayward/graphviz,ellson/graphviz,MjAbuz/graphviz,ellson/graphviz,tkelman/graphviz,pixelglow/graphviz,MjAbuz/graphviz,BMJHayward/graphviz,BMJHayward/graphviz,ellson/graphviz,BMJHayward/graphviz,tkelman/graphviz,pixelglow/graphviz,jho1965us/graphviz,BMJHayward/graphviz,tkelman/graphviz,kbrock/graphviz,tkelman/graphviz,MjAbuz/graphviz,jho1965us/graphviz,MjAbuz/graphviz,pixelglow/graphviz,BMJHayward/graphviz,BMJHayward/graphviz,jho1965us/graphviz,kbrock/graphviz,pixelglow/graphviz,kbrock/graphviz,ellson/graphviz,jho1965us/graphviz,jho1965us/graphviz,ellson/graphviz,MjAbuz/graphviz,kbrock/graphviz,pixelglow/graphviz,pixelglow/graphviz,jho1965us/graphviz,MjAbuz/graphviz,BMJHayward/graphviz,MjAbuz/graphviz,tkelman/graphviz,kbrock/graphviz,kbrock/graphviz,ellson/graphviz,MjAbuz/graphviz,ellson/graphviz,tkelman/graphviz,kbrock/graphviz,BMJHayward/graphviz,MjAbuz/graphviz,pixelglow/graphviz,MjAbuz/graphviz,ellson/graphviz,ellson/graphviz,kbrock/graphviz,kbrock/graphviz,tkelman/graphviz,jho1965us/graphviz,BMJHayward/graphviz,pixelglow/graphviz,MjAbuz/graphviz,kbrock/graphviz