hw/misc/aspeed_hace: Fix buffer overflow in has_padding function

hw/misc/aspeed_hace: Fix buffer overflow in has_padding function

The maximum padding size is either 64 or 128 bytes and should always be smaller
than "req_len". If "padding_size" exceeds "req_len", then
"req_len - padding_size" underflows due to "uint32_t" data type, leading to a
large incorrect value (e.g., `0xFFXXXXXX`). This causes an out-of-bounds memory
access, potentially leading to a buffer overflow.

Added a check to ensure "padding_size" does not exceed "req_len" before
computing "pad_offset". This prevents "req_len - padding_size" from underflowing
and avoids accessing invalid memory.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Fixes: 5cd7d8564a8b563da724b9e6264c967f0a091afa ("aspeed/hace: Support AST2600 HACE ")
Link: https://lore.kernel.org/qemu-devel/20250321092623.2097234-3-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>