staging: rtl8188eu: avoid a null dereference on pmlmepriv

There is a check on pmlmepriv before dereferencing it when
vfree'ing pmlmepriv->free_bss_buf however the previous call
to rtw_free_mlme_priv_ie_data deferences pmlmepriv causing
a null pointer deference if it is null.  Avoid this by also
calling rtw_free_mlme_priv_ie_data if the pointer is non-null.

Detected by CoverityScan, CID#1230262 ("Dereference before null check")
Fixes: 7b464c9f ("staging: r8188eu: Add files for new driver - part 4")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>