usb ethernet gadget, workaround network stack API glitch

Another workaround for the glitch in the network layer, whereby one call
ignores the (otherwise kernel-wide) convention that free() calls should
not oops when passed nulls.  This code already handles that API glitch in
most other paths.

From: Erik Hovland <erik@hovland.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>