[MachO] Fix uninitialized field bug found on Windows.

n_desc field in MachO string table was not initialized. On Unix,
test/darwin/hello-world.objtxt did not fail because I think an nlist object
is always allocated to a fresh heap initialized with zeros. On Windows,
uninitialized fields are filled with 0xCC when compiled with /GZ. Because
of that the test was failing on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193909 91177308-0d34-0410-b5e6-96231b3b80d8