When ObjectFileMachO::ParseSections() notices that it has a truncated file, zero out the

SectionList so we don't try to do anything with this file.  Currently we end up crashing
later in the debug session when we read past the end of the file -- this at least gets us
closer with something like ProcessMachCore printing "error: core file has no sections".
<rdar://problem/13468295>

llvm-svn: 179152