fix fencepost

Reference: /n/sources/patch/applied/db-input
Date: Wed Oct  3 20:17:51 CES 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/db/input.c	Wed Oct  3 20:17:38 2012
+++ /sys/src/cmd/db/input.c	Wed Oct  3 20:17:35 2012
@@ -46,12 +46,13 @@
 int
 readrune(int fd, Rune *r)
 {
-	char buf[UTFmax];
+	char buf[UTFmax+1];
 	int i;
 
 	for(i=0; i<UTFmax && !fullrune(buf, i); i++)
 		if(read(fd, buf+i, 1) <= 0)
 			return -1;
+	buf[i] = 0;
 	chartorune(r, buf);
 	return 1;
 }