[llvm-objcopy] [COFF] Fix writing object files without symbols/string table

Previously, this was broken - by setting PointerToSymbolTable to zero
but still actually writing the string table length, the object file
header was corrupted.

Differential Revision: https://reviews.llvm.org/D56584

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350926 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed