This has been there for ages, but the assertion makes no sense in the
context of the test immediately preceding it. This caused an abort()
when in -I dts -O dts mode with the right sort of internal labels in a
string property value.
Add a testcase for this and another candidate edge case (though this one
we already get right).
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>