fix leaks and improper return values in allocscreen()

Reference: /n/atom/patch/applied2013/allocscreenleak
Date: Wed Jun 19 06:42:29 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libdraw/window.c	Wed Jun 19 06:42:13 2013
+++ /sys/src/libdraw/window.c	Wed Jun 19 06:42:13 2013
@@ -20,27 +20,26 @@
 		return 0;
 	}
 	s = malloc(sizeof(Screen));
-	if(s == 0)
-		return 0;
-	SET(id);
+	if(s == nil)
+		return nil;
 	for(try=0; try<25; try++){
 		/* loop until find a free id */
 		a = bufimage(d, 1+4+4+4+1);
-		if(a == 0){
-			free(s);
-			return 0;
-		}
+		if(a == 0)
+			break;
 		id = ++screenid;
 		a[0] = 'A';
 		BPLONG(a+1, id);
 		BPLONG(a+5, image->id);
 		BPLONG(a+9, fill->id);
 		a[13] = public;
-		if(flushimage(d, 0) != -1){
-			werrstr("");
-			break;
-		}
+		if(flushimage(d, 0) != -1)
+			goto found;
 	}
+	free(s);
+	return nil;
+found:
+	werrstr("");
 	s->display = d;
 	s->id = id;
 	s->image = image;