Fixed attaching of a stack to an error object.

Fixed attaching of a stack to an error object.

This problem is similar to previous commits.  When
njs_error_stack_attach() accepted the value as a pointer to vm->retval
that value might be changed as a side effert of njs_error_stack_new()
evaluation.  This may result in a garbage value for
njs_object(value) expression.

The workaround fix is to make a copy of vm->retval to ensure its
intergrity and to preserve it as a retval. The proper fix is to
eliminate vm->retval altogether.

This fixes #612, #613, #616 issues on Github.