Fix a copy and paste error that would cause a crash.
Summary: The wrong variable was being checked for an error, which mean a llvm::Error went unchecked and crashes dsymutil. Discovered this when trying to feed an ELF file to "dsymutil --update" and running into the crash.
Reviewers: aprantl, JDevlieghere
Differential Revision: https://reviews.llvm.org/D75777