The current check is still always false when the old check was, but
additionally it will not disable the interrupts when they were never
enabled (which shouldn't matter much, but this is more consistent).
In this case, SoftwareSerial::begin will not have enabled the
interrupts, so better not allow the SoftwareSerial instance to enter the
listening state either.
Before enabling interupts, begin would see if the given receive pin
actually has an associated PCINT register. If not, the interrupts would
not be enabled.
Now, the same check is done, but when no register is available, the rx
parameters are not loaded at all (which in turn prevents the interrupt
from being enabled). This allows all code to use the same "is rx
enabled" (which will be added next).
