[MachO] Add out-of-bounds check to MachOObjectFile.cpp

This is a followup to rL333496.

Differential Revision: https://reviews.llvm.org/D47544

llvm-svn: 333929
diff --git a/llvm/lib/Object/MachOObjectFile.cpp b/llvm/lib/Object/MachOObjectFile.cpp
index 50dd476..731ff4e 100644
--- a/llvm/lib/Object/MachOObjectFile.cpp
+++ b/llvm/lib/Object/MachOObjectFile.cpp
@@ -107,6 +107,7 @@
 }
 
 static const char *getPtr(const MachOObjectFile &O, size_t Offset) {
+  assert(Offset <= O.getData().size());
   return O.getData().data() + Offset;
 }