verityutils: fix a memory leak

this CL fixes a leak of malloc'ed memory, and introduces a tool that
makes malloc'ing this memory going forward a bit easier.

caught by the static analyzer:
> system/security/ondevice-signing/VerityUtils.cpp:189:71: warning:
Potential leak of memory pointed to by 'd' [clang-analyzer-unix.Malloc]

Bug: None
Test: TreeHugger
Change-Id: I8fe9dc047af729b0f5730ec1844b3902225fa73e
1 file changed