Don't allocate memory for an error message on success.
Don't allocate memory for an error message on success.

This takes memory allocations when linking clang-fsds from 342.08MB to
320.04MB.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@319918 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld