Fix getting of extra descriptors

Also add a test to check that getting of extra descriptors works
(requires a USB device with extra descriptors, such as a UVC webcam,
which has at least one InterfaceAssociation descriptor).

Author: Rich Barlow

usb1/libusb1.py

@@ -1103,7 +1103,7 @@ def get_extra(descriptor):
     result = []
     extra_length = descriptor.extra_length
     if extra_length:
-        extra = buffer_at(descriptor.extra.value, extra_length)
+        extra = buffer_at(descriptor.extra, extra_length)
         append = result.append
         while extra:
             length = _string_item_to_int(extra[0])

usb1/testUSB1.py

@@ -244,15 +244,15 @@ def exceptionHandler(exc):
             self.assertTrue(exception_list, exception_list)
             self.assertTrue(poller.is_alive())
 
-    @staticmethod
-    def testDescriptors():
+    def _testDescriptors(self, get_extra=False):
         """
         Test descriptor walk.
         Needs any usb device, which won't be opened.
         """
         with USBContext() as context:
             device_list = context.getDeviceList(skip_on_error=True)
             found = False
+            seen_extra = False
             for device in device_list:
                 device.getBusNumber()
                 device.getPortNumber()
@@ -262,12 +262,26 @@ def testDescriptors():
                     for endpoint in settings:
                         pass
                 for configuration in device.iterConfigurations():
+                    if get_extra and len(configuration.getExtra()) > 0:
+                        seen_extra = True
                     for interface in configuration:
                         for settings in interface:
+                            if get_extra and len(settings.getExtra()) > 0:
+                                seen_extra = True
                             for endpoint in settings:
+                                if get_extra and len(endpoint.getExtra()) > 0:
+                                    seen_extra = True
                                 found = True
             if not found:
                 raise unittest.SkipTest('descriptor walk test did not complete')
+            if get_extra and not seen_extra:
+                raise unittest.SkipTest('did not see any extra descriptors')
+
+    def testDescriptors(self):
+        self._testDescriptors()
+
+    def testDescriptorsWithExtra(self):
+        self._testDescriptors(get_extra=True)
 
     def testDefaultEnumScope(self):
         """