Do not allow negative interrupt pins

[baruch]

NOT_AN_INTERRUPT is defined as -1 and might be passed to the function if used as a parameter for the attachInterrupt and detechInterrupt functions.

[MCUdude]

I agree that allowing -1 to be passed to the function is not a good idea, but the PR adds one more query. It all adds up, so I rather prefer to use.

if(interruptNum == EXTERNAL_INTERRUPT_0)
{
// code
}

[baruch]

Because the code is compiled with optimizations and assuming that the input is a constant the actual comparison is eliminated and also the entire code is eliminated if the incorrect value is passed.

That said, your comparison method is just as valid for this case (single pin supporting INT0).

As an aside, my personal preference is to avoid the nested if with:

if (interruptNum != EXTERNAL_INTERRUPT_0)
    return;

at the start, this also means you only need a single ifdef SAFEMODE as well.

[MCUdude]

Good idea! Will commit this change very soon 👍