Update src/goto-programs/goto_convert.cpp

Author: tautschnig

src/goto-programs/goto_convert.cpp

@@ -1834,7 +1834,9 @@ bool goto_convertt::get_string_constant(
           if(to_integer(to_constant_expr(*it), int_value))
             return true;
 
-          unsigned long i = integer2ulong(int_value);
+          const auto i = numeric_cast<std::size_t>(*it);
+          if(!i.has_value())
+            return true;
 
           if(i!=0) // to skip terminating 0
             result+=static_cast<char>(i);