Check if object is nil before writing trace

We were accessing fields of the expression without first checking that
it is not nil which could lead to invalid memory access

Author: romainbrenguier

src/goto-programs/goto_trace.cpp

@@ -91,7 +91,7 @@ void goto_trace_stept::output(
 
   out << "\n";
 
-  if(pc->is_other() || pc->is_assign())
+  if((pc->is_other() && lhs_object.is_not_nil()) || pc->is_assign())
   {
     irep_idt identifier=lhs_object.get_object_name();
     out << "  " << from_expr(ns, identifier, lhs_object.get_original_expr())
@@ -386,14 +386,8 @@ void show_goto_trace(
       break;
 
     case goto_trace_stept::typet::CONSTRAINT:
-      UNREACHABLE;
-      break;
-
     case goto_trace_stept::typet::SHARED_READ:
     case goto_trace_stept::typet::SHARED_WRITE:
-      UNREACHABLE;
-      break;
-
     default:
       UNREACHABLE;
     }