Mark the end of read_bin_goto_object with UNREACHABLE

The proceding control flow always returns before this point therefore we
can use `UNREACHABLE` to make the control flow more explicit.

Author: thomasspriggs

src/goto-programs/read_bin_goto_object.cpp

@@ -279,5 +279,5 @@ bool read_bin_goto_object(
     }
   }
 
-  return false;
+  UNREACHABLE;
 }