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pc/rtc: make sure string is NUL terminates for rtc write

--- a/sys/src/9/pc/devrtc.c

+++ b/sys/src/9/pc/devrtc.c

@@ -227,26 +227,25 @@

 	char *a, *start;

 	Rtc rtc;

 	ulong secs;

+	char *cp, sbuf[32];

 	uchar bcdclock[Nbcd];

-	char *cp, *ep;

 	ulong offset = off;

 	if(offset!=0)

 		error(Ebadarg);

-

 	switch((ulong)c->qid.path){

 	case Qrtc:

 		/*

 		 *  read the time

 		 */

-		cp = ep = buf;

-		ep += n;

-		while(cp < ep){

-			if(*cp>='0' && *cp<='9')

+		if(n >= sizeof(sbuf))

+			error(Ebadarg);

+		strncpy(sbuf, buf, n);

+		sbuf[n] = '\0';

+		for(cp = sbuf; *cp != '\0'; cp++)

+			if(*cp >= '0' && *cp <= '9')

 				break;

-			cp++;

-		}

 		secs = strtoul(cp, 0, 0);

 		/*