verityutils: fix a memory leak

`d` is never freed here. use a unique_ptr to ensure that happens in the
future.

caught by the static analyzer:
> system/security/ondevice-signing/VerityUtils.cpp:88:26: warning:
Potential leak of memory pointed to by 'd' [clang-analyzer-unix.Malloc]

Bug: None
Test: TreeHugger
Change-Id: I0158eae424a024046089d86fe3c1f38efbfa9a2b
1 file changed