gpio.c: check that real gpio did initialise correctly

On platforms without muxing perform a little more checking when the 'real'
linux/sysfs gpio is opened. We don't expect this to fail but on platforms where
the linux gpios can be changed depending on pinctrl/bios configuration this
might not always be the case.

Signed-off-by: Henry Bruce <henry.bruce@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/gpio/gpio.c

@@ -75,6 +75,10 @@ mraa_gpio_init(int pin)
     }
 
     mraa_gpio_context r = mraa_gpio_init_raw(plat->pins[pin].gpio.pinmap);
+    if (r == NULL) {
+        syslog(LOG_CRIT, "gpio: mraa_setup_gpio(%d) returned error %d", pin, r);
+        return NULL;
+    }
     r->phy_pin = pin;
 
     if (advance_func->gpio_init_post != NULL) {