spdm: Fix memory leaks

Some code in the library declares MaybeUnit and write to them through
Out pointers, but doesn't call assume_init() afterwards. This works
because methods that write to Out pointers usually return mut ref to the
same object for convenience. But that might cause memory leak if the
underlying type of MaybeUninit is heap allocated. This CL makes all
places that the underlying type of MaybeUninit might be heap allocated
explicitly assume_init().

BUG=b:341936780
TEST=cargo test with asan

Change-Id: I8dcc164d8a13466ae07b162417460dc7d82e5b96
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/spdm/+/5563711
Reviewed-by: Vadim Sukhomlinov <sukhomlinov@google.com>
Commit-Queue: Howard Yang <hcyang@google.com>
Tested-by: Howard Yang <hcyang@google.com>
7 files changed