Per the data sheet, the absolute maximum amount of current an i/o port pin should ever sink or source (to prevent damage, not for normal operation) is 40mA. That said, I'm reading this as one needs to short-circuit protect all i/o port pins.
I'm now seeing this as (assuming 5v drive),
5v --- R --- short-to-GND
(only considering 5v-linked pins)
35mA < 5/R (incl. safety margin)
R > 5v/35mA
R > 143 ohms
in series with every pin.
Also, we're looking at
P = 5^2/143 = 25/143 = 174mW, so they'll have to be 1/4W resistors also, lest they become fuses.
Does this seem reasonable, the right approach?