iommu/omap: fix NULL pointer dereference

Fix this:

root@omap4430-panda:~# cat /debug/iommu/ducati/mem
[   62.725708] Unable to handle kernel NULL pointer dereference at virtual addre
ss 0000001c
[   62.725708] pgd = e6240000
[   62.737091] [0000001c] *pgd=a7168831, *pte=00000000, *ppte=00000000
[   62.743682] Internal error: Oops: 17 [#1] SMP
[   62.743682] Modules linked in: omap_iommu_debug omap_iovmm virtio_rpmsg_bus o
map_remoteproc remoteproc virtio_ring virtio mailbox_mach mailbox
[   62.743682] CPU: 0    Not tainted  (3.3.0-rc1-00265-g382f84e-dirty #682)
[   62.743682] PC is at debug_read_mem+0x5c/0xac [omap_iommu_debug]
[   62.743682] LR is at 0x1004
[   62.777832] pc : [<bf033178>]    lr : [<00001004>]    psr: 60000013
[   62.777832] sp : e72c7f40  ip : c0763c00  fp : 00000001
[   62.777832] r10: 00000000  r9 : 00000000  r8 : e72c7f80
[   62.777832] r7 : e6ffdc08  r6 : bed1ac78  r5 : 00001000  r4 : e7276000
[   62.777832] r3 : e60f3460  r2 : 00000000  r1 : e60f38c0  r0 : 00000000
[   62.777832] ...