avoid crashing in einit when initdraw fails

Reference: /n/sources/patch/applied/plotcrash
Date: Wed Dec 23 17:59:46 CET 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/plot/libplot/machdep.c	Wed Dec 23 17:59:41 2009
+++ /sys/src/cmd/plot/libplot/machdep.c	Wed Dec 23 17:59:40 2009
@@ -67,7 +67,8 @@
 	int dx, dy;
 	USED(s);
 	if(first){
-		initdraw(0,0,"plot");
+		if(initdraw(0,0,"plot") == -1)
+			sysfatal("initdraw: %r");
 		einit(Emouse);
 		clipminx=mapminx=screen->r.min.x+4;
 		clipminy=mapminy=screen->r.min.y+4;