[lld][MachO] Always reserve space for the empty string in the mach-o symbol

table.

The first entry in the MachO symbol table is always the empty string: make sure
we reserve space for it, or we will overflow the symbol table by one byte.

No test case - this manifests as an occasional memory error. In the near future
I hope to set up a bot building and runnnig LLD with sanitizers - that should
catch future instances of this issue.

llvm-svn: 255178