Fix an off-by-2 error in the source output code

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>