staging: rtl8188eu: Fix null pointer dereference on free_netdev call

staging: rtl8188eu: Fix null pointer dereference on free_netdev call

An unregister_netdev call checks if pnetdev is null, hence a later
call to free_netdev can potentially be passing a null pointer, causing
a null pointer dereference. Avoid this by adding a null pointer check
on pnetdev before calling free_netdev.

Fixes: 1665c8fdffbb ("staging: rtl8188eu: use netdev routines for private data")
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210324152135.254152-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>