yaml2obj might crash on invalid input when unable to parse the YAML.
Recently a crash with a very similar nature was fixed for an empty files.
This patch revisits the fix and does it in yaml::Input instead.
It seems to be more correct way to handle such situation.
With that crash for invalid inputs is also fixed now.
Differential revision: https://reviews.llvm.org/D61059
llvm-svn: 359178
George Rimar