Check for valid pin properly (fixes #4605)

Fix for `pinMode()`, `digitalWrite()`, `digitalRead()` (issue #4605).
Current behavior: look up pin number in `digital_pin_to_port_PGM[]` and then check if it returned `NOT_A_PIN`.  Causes undefined behavior if provided `pin` number is out of the range of `digital_pin_to_port_PGM[]`.
Proposed behavior (from issue #4605): check if `pin` is within the valid range of `digital_pin_to_port_PGM[]`, and THEN look it up.

Additionally, remove second check for `port` not being `NOT_A_PIN` (which was useful for boards where the pin numbering skips some numbers).  This can still be achieved by making `bit = digitalPinToBitMask(pin)` be 0 for invalid pins, which causes further bitwise operations such as `*reg &= ~bit;` and `*out |= bit;` to not actually modify the value of the register.  (This removal makes the operation complete a bit faster for valid pins and slower for invalid pins, which I think is a good trade; plus it saves binary size.)

Author: cousteaulecommandant

Diff for: hardware/arduino/avr/cores/arduino/wiring_digital.c

@@ -26,14 +26,16 @@
 #include "wiring_private.h"
 #include "pins_arduino.h"
 
+#define PIN_MAX (sizeof digital_pin_to_port_PGM / sizeof *digital_pin_to_port_PGM)
+
 void pinMode(uint8_t pin, uint8_t mode)
 {
-	uint8_t bit = digitalPinToBitMask(pin);
-	uint8_t port = digitalPinToPort(pin);
+	if (pin >= PIN_MAX) return;
+	
+	uint8_t bit = digitalPinToBitMask(pin); // bit mask, or 0 for invalid pins
+	uint8_t port = digitalPinToPort(pin); // must be a valid port (even for invalid pins)
 	volatile uint8_t *reg, *out;
 
-	if (port == NOT_A_PIN) return;
-
 	// JWS: can I let the optimizer do this?
 	reg = portModeRegister(port);
 	out = portOutputRegister(port);
@@ -137,13 +139,13 @@ static void turnOffPWM(uint8_t timer)
 
 void digitalWrite(uint8_t pin, uint8_t val)
 {
+	if (pin >= PIN_MAX) return;
+	
 	uint8_t timer = digitalPinToTimer(pin);
 	uint8_t bit = digitalPinToBitMask(pin);
 	uint8_t port = digitalPinToPort(pin);
 	volatile uint8_t *out;
 
-	if (port == NOT_A_PIN) return;
-
 	// If the pin that support PWM output, we need to turn it off
 	// before doing a digital write.
 	if (timer != NOT_ON_TIMER) turnOffPWM(timer);
@@ -164,12 +166,12 @@ void digitalWrite(uint8_t pin, uint8_t val)
 
 int digitalRead(uint8_t pin)
 {
+	if (pin >= PIN_MAX) return LOW;
+	
 	uint8_t timer = digitalPinToTimer(pin);
 	uint8_t bit = digitalPinToBitMask(pin);
 	uint8_t port = digitalPinToPort(pin);
 
-	if (port == NOT_A_PIN) return LOW;
-
 	// If the pin that support PWM output, we need to turn it off
 	// before getting a digital reading.
 	if (timer != NOT_ON_TIMER) turnOffPWM(timer);