Prevent arg from being used any further

Patch edited by David van Moolenbroek.

Change-Id: Ie25faffea359e735145dbb4ae2bd2d213488952b

minix/kernel/utility.c

@@ -31,6 +31,7 @@ void panic(const char *fmt, ...)
 	printf("kernel panic: ");
   	va_start(arg, fmt);
 	vprintf(fmt, arg);
+	va_end(arg);
 	printf("\n");
   }