[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

llvm/lib/Object/COFFObjectFile.cpp

@@ -1062,7 +1062,7 @@ COFFObjectFile::getSectionContents(const coff_section *Sec,
   // In COFF, a virtual section won't have any in-file
   // content, so the file pointer to the content will be zero.
   if (Sec->PointerToRawData == 0)
-    return object_error::parse_failed;
+    return std::error_code();
   // The only thing that we need to verify is that the contents is contained
   // within the file bounds. We don't need to make sure it doesn't cover other
   // data, as there's nothing that says that is not allowed.