Fix the parsing of images without hashtree descriptor

It's possible that the an image doesn't contain a hashtree
descriptor. And the current raises a segfault when we attempt
to validate and copy a non existing descriptor.

Bug: 157778739
Test: call fec_open on dtbo image.
Change-Id: Ieb5acf016ceeb547f2b23de43d4276ea4d013a6f
(cherry picked from commit 90e7155bdd10039448bbc5ee4636f14c0119718e)
1 file changed