Fix some rather confusing indentation and control flow in the errno

Fix some rather confusing indentation and control flow in the errno
printing routine. This is made harder to see due to the surprising
formatting, inconsistent brace usage, and repeated conditions that all
test the same thing.

The only "consequence" of this bug is re-assigning 'str' to an empty
string when computing the error string for an error number of 0 in the
event of a non-GNU strerror_r routine. So, nothing to see here other
than cleanup. It did help me find PR17055 in clang-format though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189734 91177308-0d34-0410-b5e6-96231b3b80d8