Fixed a crash in llvm-mc for Mach-O when a symbol difference expression uses a

symbol from an assignment.  In this case the symbol did not have a fragment so
MCObjectWriter::IsSymbolRefDifferenceFullyResolved() should not have been
calling IsSymbolRefDifferenceFullyResolvedImpl() with a NULL fragment and should
just have returned false in that case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149442 91177308-0d34-0410-b5e6-96231b3b80d8