forsyth: nit: sync overflow check with arg string building

Reference: /n/atom/patch/applied/ccargchknit
Date: Sun Aug 16 01:30:52 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/cc/dpchk.c	Sun Aug 16 01:30:43 2015
+++ /sys/src/cmd/cc/dpchk.c	Sun Aug 16 01:30:44 2015
@@ -75,9 +75,9 @@
 	nstar = 0;
 	for(;;) {
 		s += chartorune(&c, s);
-		fmt += runetochar(fmt, &c);
 		if(c == 0 || c >= nelem(flagbits))
 			break;
+		fmt += runetochar(fmt, &c);
 		f = flagbits[c];
 		switch(f) {
 		case Fnone: