[yaml2obj][ELF] Ensure more fields are zero'd.

I was spotting garbage in the output. I'd like to just zero the entire
ELFYAML::Section to be sure, but it contains non-POD types. (I'm also
trying to avoid bloating the ELFYAML::Foo classes with a bunch of
constructor code).

No test, since this is by its very nature unpredictable. I'm pretty sure
that one of the sanitizers would catch it immediately though.

llvm-svn: 184160