cpuss_dump: fix potential overflow for core_reg_num

In practise, core_reg_num should not overflow. But in
theory, this value can overflow and system_regs_input_index
also can overflow. So add overflow checks for these two.

Change-Id: I27e9544f3769c097d00d8660b7eb8c6cd29bb6de
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>