verity: Do not increment data when it is nullptr.

verity: Do not increment data when it is nullptr.

Address a segfault introduced in
d6dc877032c65768b3c6737156d3142f32cc9984

As soon as leftover_ becomes non-empty, the data pointer is incremented
and will now be an invalid pointer in case it was null previously.
The subsequent check in HashBlocks() will now encounter a non-null data
ptr and pass it to HashBlock(), triggering a segfault.

Test: Manually build and boot an image on which this crashed before.
Test: build_verity_tree_test
Change-Id: I324ff6103c9d35fb290c4e96fb5009ee365a0249
Signed-off-by: MarijnS95 <marijns95@gmail.com>
(cherry picked from commit c82514bd034f214b16d273b10c676dd63a9e603b)