While doing delayed expansion of escape sequences in macro arguments,

While doing delayed expansion of escape sequences in macro arguments,
correctly check for failure of the in-place expansion function.
If an argument not only does recursive delayed expansion
but infinitely recursive delayed expansion, this bug could
result in an ESCAPE_EXPAND assertion failure.

Thanks to Eric van Gyzen <vangyzen at FreeBSD> for finding this bug
by inspecting FreeBSD source code.