USBHOST: fix call back

Author: jamike

features/unsupported/USBHost/USBHost/USBDeviceConnected.cpp

@@ -94,7 +94,7 @@ void USBDeviceConnected::init(uint8_t hub_, uint8_t port_, bool lowSpeed_) {
 
 void USBDeviceConnected::disconnect() {
     for(int i = 0; i < MAX_INTF; i++) {
-        intf[i].detach.call();
+        if (intf[i].detach) intf[i].detach.call();
     }
     init();
 }

features/unsupported/USBHost/USBHost/USBEndpoint.h

@@ -104,7 +104,8 @@ class USBEndpoint
     * Call the handler associted to the end of a transfer
     */
     inline void call() {
-        rx.call();
+        if (rx)
+            rx.call();
     };