[COFF] Gracefully handle empty .drectve sections

Running `llvm-readobj -coff-directives msvcrt.lib` resulted in this error:

    Invalid data was encountered while parsing the file

This happened because some of the object files in the archive have empty
`.drectve` sections. These empty sections result in a `parse_failed` error being
returned from `COFFObjectFile::getSectionContents()`, which in turn caused
`llvm-readobj` to stop. With this change, `getSectionContents` now returns
success, and like before the resulting array is empty.

Patch by Dave Lee.

Differential Revision: https://reviews.llvm.org/D32652

llvm-svn: 303014