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

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75777