Don't read one command past the end.

Thanks to Evgeniy Stepanov for reporting this.

It might be a good idea to add a command iterator abstraction to MachO.h, but
this fixes the bug for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179848 91177308-0d34-0410-b5e6-96231b3b80d8