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kernel: be more careful about argc for /dev/drivers writes

Not crashing on 'chdev &' is important.

--- a/sys/src/9/port/devcons.c

+++ b/sys/src/9/port/devcons.c

@@ -705,18 +705,22 @@

 			error(Ebadarg);

 		if(ct->index != CMchdev)

 			error(Ebadarg);

-		if(cb->nf < 2 || cb->nf > 3)

+		if(cb->nf == 1)

 			error(Ebadarg);

 		invert = 1;

-		a = cb->f[2];

+		a = "";

 		switch(cb->f[1][0]){

 		case '&':

+			if(cb->nf != 3)

+				error(Ebadarg);

+			a = cb->f[2];

 			if(cb->f[1][1] == '~')

 				invert--;

 			break;

 		case '~':

-			a = "";

+			if(cb->nf != 2)

+				error(Ebadarg);

 			break;

 		default:

 			error(Ebadarg);