firmware_class: avoid double free

The error exit path in request_firmware frees the allocated struct firmware *firmware, which is good. What is not so good is that the value of firmware has already been copied out to the caller as *firmware_p. The risk is that the caller will pass this to release_firmware, a double free. This is exactly what will happen if the caller copied the example code

         if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
                copy_fw_to_device(fw_entry->data, fw_entry->size);
         release(fw_entry);

from the firmware documentation.

@@ -441,6 +441,7 @@
 
 error_kfree_fw:
 	kfree(firmware);
+	*firmware_p = NULL;
 out:
 	return retval;
 }