[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