Follow-up fix to r348811 for null Errors (which is the case for end iterators)

Not sure how I missed that in my testing, but obvious enough - this
causes segfaults when attempting to dereference the Error in end
iterators.

llvm-svn: 348814

include/llvm/Object/ELFTypes.h

@@ -676,9 +676,9 @@ public:
     return *this;
   }
   bool operator==(Elf_Note_Iterator_Impl Other) const {
-    if (!Nhdr)
+    if (!Nhdr && Other.Err)
       (void)(bool)(*Other.Err);
-    if (!Other.Nhdr)
+    if (!Other.Nhdr && Err)
       (void)(bool)(*Err);
     return Nhdr == Other.Nhdr;
   }