STM USBHALHost: Fix NULL pointer dereference

On STM32F746G Discovery boards, the USB OTG HS port does not have a
dedicated GPIO for controlling the USB VBUS.

This change fixes HardFault (NULL pointer dereference) that triggered
when such USB host port was used.

Author: desowin

features/unsupported/USBHost/targets/TARGET_STM/USBHALHost_STM.h

@@ -122,10 +122,17 @@ static gpio_t gpio_powerpin;
 
 void usb_vbus(uint8_t state)
 {
-    if (state == 0) {
-        gpio_write(&gpio_powerpin, USB_POWER_OFF);
-    } else {
-        gpio_write(&gpio_powerpin, USB_POWER_ON);
+    if (gpio_powerpin.reg_set && gpio_powerpin.reg_clr)
+    {
+        if (state == 0) {
+            gpio_write(&gpio_powerpin, USB_POWER_OFF);
+        } else {
+            gpio_write(&gpio_powerpin, USB_POWER_ON);
+        }
+    }
+    else
+    {
+        /* The board does not have GPIO pin to control usb supply */
     }
     wait(0.2);
 }