cris/kgdb: Fix buffer overflow in getpacket()

arch/cris/arch-v10/kernel/kgdb.c: In function 'handle_exception':
arch/cris/arch-v10/kernel/kgdb.c:534:17: warning: array subscript is above array bounds [-Warray-bounds]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>