Use a safety belt for cases where corrupted narg can be passed to the

ktrsyscall(). print_number() does decrement the number of arguments,
leading to infinite loops for negative values.

Reported by:	Patrick Lamaiziere <patpr at davenulle dot org>,
		Jonathan Pascal <jkpyvxmzsa at mailinator dot com>
Submitted by:	jh
PR:		bin/120055, kern/119564
MFC:		1 week

usr.bin/kdump/kdump.c

@@ -799,7 +799,7 @@ ktrsyscall(struct ktr_syscall *ktr)
 				narg--;
 			}
 		}
-		while (narg) {
+		while (narg > 0) {
 			print_number(ip,narg,c);
 		}
 		(void)putchar(')');