iommu: msm: Add null-pointer check in page table free code

There is a missing null-pointer check in the code that frees page tables
for long descriptor format page table (LPAE). This can cause a crash if
a client had mapped to the end of a 1GB window but did not have a
mapping in the next 1GB and then tries to unmap.

Change-Id: I9628e8f365fc4933080337dbdd4b2ff2f0d2f068
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>