[llvm-objcopy] [COFF] Do not patch debug entries if PointerToRawData is zero

Fix an edge case missed by https://reviews.llvm.org/D78921.  For e.g.,
the Repro debug entry (generated with the /Brepro linker flag) does not
have a debug-directory payload.  Do not attempt to patch Debug entries
without a payload.

Differential Revision: https://reviews.llvm.org/D107324