sh4: r2d fix no ide/net case

Fix invalid access/crash when there is no IDE device or NET device.

Signed-off-by: Takashi YOSHII <takasi-y@ops.dti.ne.jp>
Tested-by: Shin-ichiro KAWASAKI <kawasaki@juno.dti.ne.jp>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6673 c046a42c-6fe2-441c-8c8c-71466251a162
