[mach-o] don't assume all sections have symbols

We were trying to examine the first symbol in a section that we wanted to
atomize by symbols, even when there wasn't one. Instead, we should make the
initial anonymous symbol cover the entire section in that situation.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211681 91177308-0d34-0410-b5e6-96231b3b80d8