[ObjCopy][NFC] Remove unneeded zero initialization

[MaskRay]

Summary

getNewMemBuffer has called memset.

llvm/lib/ObjCopy/MachO/MachOWriter.cpp

 
 Error MachOWriter::write() {
   size_t TotalSize = totalSize();
   Buf = WritableMemoryBuffer::getNewMemBuffer(TotalSize);
   if (!Buf)
     return createStringError(errc::not_enough_memory,
                              "failed to allocate memory buffer of " +
                                  Twine::utohexstr(TotalSize) + " bytes");
-  memset(Buf->getBufferStart(), 0, totalSize());
   writeHeader();
   writeLoadCommands();
   writeSections();
   writeTail();
 
   // TODO: Implement direct writing to the output stream (without intermediate
   // memory buffer Buf).
   Out.write(Buf->getBufferStart(), Buf->getBufferSize());
   return Error::success();
 }