have strtochan return zero if the depth is invalid.

Notes:
Wed May 31 10:00:28 EDT 2006 rsc
    please resubmit this with d and c out of the
    for loop initialization.  since they're not part
    of the rest of the for(;;) expressions, they
    don't belong there.  just use a normal initialization
    and leave the loop as a while loop.
    
    thanks.


Reference: /n/sources/patch/sorry/strtochan
Date: Sun May 28 23:29:38 CES 2006
Signed-off-by: quanstro@quanstro.net
Reviewed-by: rsc

--- /sys/src/libdraw/chan.c	Sun May 28 23:27:23 2006
+++ /sys/src/libdraw/chan.c	Sun May 28 23:27:20 2006
@@ -41,23 +41,25 @@
 {
 	char *p, *q;
 	ulong c;
-	int t, n;
+	int t, n, d;
 
-	c = 0;
 	p=s;
 	while(*p && isspace(*p))
 		p++;
 
-	while(*p && !isspace(*p)){
+	for(d = 0, c = 0; *p && !isspace(*p);){
 		if((q = strchr(channames, p[0])) == nil) 
 			return 0;
 		t = q-channames;
 		if(p[1] < '0' || p[1] > '9')
 			return 0;
 		n = p[1]-'0';
+		d += n;
 		c = (c<<8) | __DC(t, n);
 		p += 2;
 	}
+	if(d==0 || (d>8 && d%8) || (d<8 && 8%d))
+		return 0;
 	return c;
 }