fdc: force the fifo access to be in bounds of the allocated buffer

fdc: force the fifo access to be in bounds of the allocated buffer

During processing of certain commands such as FD_CMD_READ_ID and
FD_CMD_DRIVE_SPECIFICATION_COMMAND the fifo memory access could
get out of bounds leading to memory corruption with values coming
from the guest.

Fix this by making sure that the index is always bounded by the
allocated memory.

This is XSA-133 / CVE-2015-3456.

Signed-off-by: Petr Matousek <pmatouse@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
(cherry picked from commit 4de1422ea306832b6ef2cba34e9febb73dd139a7)