Fix handling of files without a symbol table.

This fixes a recent regression (r183338). Stripped elf files (like installed
crtn.o for example), are not required to have a symbol table. Handle that
correctly.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183573 91177308-0d34-0410-b5e6-96231b3b80d8